ChKn,
Jumping where you do when you recover is wrong.
As long as Marth has his double jump, he retains a lot of control in his movements off stage + can nearly always recover. Think about holding onto that jump off stage forever. Try just weaving backwards/forwards off stage a bit and just try to get to the ledge with just bubble-aerials and take whiffs / lag from them as a free opportunity to recover. Marth can be in the "1% per second bubble" on the low blast zone and with his jump still recover.
Figure out what hitting someone actually does. Untippered fairs pre-40 is usually not enough time to combo, so you should be taking defensive options rather than a second swing (you get shielded a lot here). When you get tipper aerials though, you need to go in. Tipper fairing MK or Sonic and then walking away feels awfully dumb. Learn how far people travel and just follow people's DI. There's no jumps, there's no b-reverses. When someone gets hit by a move there's on average one and a half directions someone will travel in.
Swing later.
Everyone likes to punish marth after swinging. Sonic and MK are pretty good examples of characters which do that. A lot of your swings are (from a logic standpoint) often completely useless and are just giving you lag + time for your opponent to react + obtain a better position. You may not feel comfortable with your reaction speed, but there's still better ways to keep strong in neutral situations. Yeah. your neutral doesn't apply pressure because you swing for no reason, you rescind after getting hits (So you don't continue pressure, so to them getting hit is fine if 2/3 times they're not getting hit).
Swinging and hitting followed by you landing is always a walk forward (for the most part). In low percent/untippered times, your immediate action after walking forward is usually shield, because they're going to try to hit you. It's obvious. It's what everyone does. Just walk forward and power shield just as they're about to reach your height coming close to the ground. Or suddenly get into a good space where they're worried about your PS and dtilt or ftilt or read their jump with a dash fair. So if you hit someone with a fair that you can't fair them again before they can hit you, you know it because you know your character, be prepared to retreat. Obviously untippered fairs at even 10% can combo, but this is based on your frame advantage due to: swinging as last as you possibly can (more frame advantage due to landing), and you'll get to "feel" when those are safe to continue from or not (untippered fair at lowish percent when I know I can follow up due to a near immediate landing is pretty sexy, up until around 40-50% it almost always combos into dash full hop fair).
Jumping is very important to Marth. It's a laggy movement. Once you're in the air as Marth, you are safe to react. But this means that you need to figure out where to jump / question the reasons why you jump like you do. Marth shouldn't get punished for jumping (because you shouldn't be jumping when they can hit you / have such a good positional advantage in the first place). Blinking + dash dance back rolls to give you more freedom to get to the spots you need to jump safely.
Starting a match and jumpng as Marth into ff-fair as a pattern, when you're about 75% of battlefield's stage away just isn't smart. It's not providing you anything. Literally nothing. The time between each jump + ff fair is like THIRTY TO FORTY FRAMES. Ganondorf is charging a warlock punch in your face. You wouldn't do that against Ganondorf. So why would you do it against a significantly better character?. (Yes ganondorf can't dash fast enough, but you get the point)
Your other weakness is landing. The more comfortable you get with the specific matchs up + their range, you'll find yourself thinking about where you can land assuming you know what they're doing (i.e. fsmash from either sonic or MK). Marth's double jump held backwards is an amazing pseudo air dodge that's gets you out FOR FREE against a lot of this landing ****. Marth's backwards double jump flattens his body like snake's bair from Marth's head (so no dangley legs). Learn that animation and how you can forward air out of it. Marth's backwards double jump + forward air is the exact same thought process as crouching and then shielding for a power shield on the ground, except you're in the air and you can retreat/backwards forwards and also fast fall.
Frame four of forward air.
It's what you need to learn the most.
Both people you played DI the same way. Both people are set up in a position where frame 4 auto tipper fair is as free as the sun, moon and the stars. So you're slightly below them, but in their face. Tons of pressure. If you're in the position where frame 4 fair will hit, you ffing that aerial will always be a significant positional advantage if they do any combination of air dodging, fast falling or jumping.
You know enough about Marth to know all the traps. You know enough about Marth to know what move is generally appropriate at around the time you're in. Now you need to get your timing on point and get better at spacing because of it. Fair to dtilt is amazing. But that fair has to actually be applying pressure in the first place.
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Match up stuff
When MK is gliding off stage, he's either good at tech skill and is able to fly past the ledge, or he's going to grab the ledge. Chances are he's the latter (Your MK is definitely the latter). Don't be afraid of gliding from off stage when he's not going above the ledge. When you play someone who doesn't grab the ledge there, then you need to be smarter/safer in other ways, but I'm sure you could figure that out if they're mixing that up + you're aware of that possibility + reaction speed.
At 2.18, I think you made a beautiful positional play. You've zoned MK towards the edge and you're in a position for down tilt to beat/trade (and considering that dashing off stage into fair is saving you 6 frames of jump time and is **** **** **** that combos from down tilt, trading with ftilt or "whiffing" in these scenarios IS NOT BAD (especially if you're comfortable with buffering things out of dtilt such as smash di/grab break mashing [but you know its ftilt, so SDI]). Instead you jumped. Not only, IMO, was it a bad jump (safety wise), you got the jump for free, but didn't even try to space fair to hit his grubby little hands while he ftilts. Spacing frame 7 of fair properly hits MK while he's ftilting in the first two hits and also covers you from shuttle loop.
Talking about recovery. 3minutes in the MK match is a really good example. You use your second jump. When you could've held it for a lot longer until you were closer to the stage. You were completely safe to do this. If you have an inkling of MK dashing off stage with a ff dair, then what the ****, react to that and fair it. But you're afraid of something your opponent isn't good enough to do yet.
MK's shuttle loop loses to spaced fairs. MK's love taking a position and then shuttle looping the moment you get too close / swing. There's your problem. Getting too close (think of hitting with frame 7 of fair) + swinging way too early.
Shielding fsmash from MK is one of those things where shield to reshield is ****.
Nearly every level of MK has an fsmash taken on shield into another attack. Usually down smash. You can't shield drop + db1 fsmash, but you can shield drop, walk slightly forward and shield. And get a free powershield on dsmash or tilts.
Sonic stuff..
yeah swinging later will help you a ton. Not trying to hit sonic while he's charging specials on the ground (although you can with DB1). You're better off trying to hit sonic in the air. Sonic wants to hit you with those specials and then auto-execution into some combos. Dodge that hit and punish their habits for what they do afterwards.
Aerials clash with sonic's up b. Getting edgeguarded by Sonic's Up B is definitely not a thing. So in other words, a planking fair will give you the safety you need to let go off the ledge and reclaim your invincibility.