Q: Valid English sentence (grammatically) or not: It is this hat that that he was wearing is certain.

A: unacceptable

Q: Valid English sentence (grammatically) or not: John seems that it is likely to win.

A: unacceptable

Q: Valid English sentence (grammatically) or not: They said that Tom wouldn't pay up, but he did go to the bank, and he did pay up.

A:
acceptable