Hi welcome to the forum. I'll answer your questions here, but also check out my video library, it'll have many answers for you on other topics you might want to learn about.

Special Abilities give them extra talent in that area. So a free kick specialist will outperform their ratings on taking free kicks. So a high rated striker will now be even better. A low rated defender still won't be worth much.

Special Abilities are nice, but I don't go crazy for them, except for the best ones, and even then, I can live without.
See my video for a full explanation and how to use them.

Form is critical, if players are averaging under 7, they usually aren't doing well.
9 = Great game, 8 = Good game, 7 = Average game 6 = Below Average game, 5 = Bad Game, 4 = Terrible game, 3 = This guy was playing drunk.

MCs and DMCs tend to get lower ratings than the rest, so anything over a 7 average is fine.
Defenders and strikers you'd like 7.2 or better.
Keepers are random, just read the commentary and decide if he is doing his job. It's not hard to tell.

My super star averages about 7.7-7.9, but this is rare, usually you won't find anyone aside from the best over 7.5. Anything higher than that means he is a star.
Good form and low stars = Good player
Bad form and high stars = Bad player
I would rather have a 2 star player that gets good form than a 6 star who gets 565665767

If your players get low form you have some options:
A: Change their positions (if they have more than 1 position available)
B: Change your formation (some players do better in certain formations)
C: Rest him for a few games (sometimes this helps)
D: Sell
I had a guy last season score 32 goals in a 451V after scoring 9 the season before in Hexagon. This season he isn't doing much in my Narrow Diamond, but my other AMC who is rated almost the same has 8 goals already.

Hope that helps!