Use of the wrong tense

Using can instead of could in a subordinate clause.

Don’t say: Ben thought he can win the prize.

ü Say: Ben thought he could win the prize.

Can changes to could in subordinate clauses, when the verb in the main clause is in the past simple tense.

Use of the wrong tense

Using will/’ll instead of would/’d in a subordinate clause.

Don’t say: He said (that) he will/’ll come tomorrow.

ü Say: He said (that) he would/’d come tomorrow.

Will/’ll changes to would/’d in subordinate clauses, when the verb in the main clause is in past tense.