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: the past tense).

For example, a phrase like this: I wasn’t sure if he understood my words.
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Phrase the Cat inviting to practise English Phrase the Cat waving

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