How to construct a compound sentence that expresses doubt?

Use if or whether (more formal) to introduce clauses after a main clause that expresses doubt.
If the action in the secondary clause happens at the same time as the action in the main clause, use the same tense (here: past simple).

example sentence: I wasn’t sure if he understood my words.
Solidify this and other rules

More rules like this

Phrase the Cat inviting to practise English Phrase the Cat waving

At you can study a brief grammar rule and start creating new phrases based on it right away.