Can impossible attack rolls still hit?

[Sirrah]

I only own the Beginner's Box so sorry if this is answered in the core rule book.

If a player cannot hit the target AC will a roll of 20 still be an automatic hit and possibly critical?

Also, if a player needs 20 to hit the target AC is 18-19 still a critical hit using a scimitar?

[Bardic Dave]

1) Yes
2) No. Only a 20 is an automatic hit.

[8 Red Wizards]

It's still an automatic hit 20 hits everything, and 1 misses everything. If you can't hit on 18 or 19 with your scimitar you can't crit.