Verb changes from past to present sometimes confuse writers. I often see mistakes like this:
Joan tiptoed to the bedroom window without waking Sam. Or so she thinks. He’s lying there perfectly still, pretending that he was asleep. INCORRECT
“Tiptoed” is past; “he’s lying there perfectly still” is present tense.
When you’re telling a story, you can’t jump back and forth between present and past. Those sentences should be written like this:
Joan tiptoed to the bedroom window without waking Sam. Or so she thought. He was lying there perfectly still, pretending that he was asleep. CORRECT
But that doesn’t mean you can’t mix tenses (past, present, future) in the same sentence. Here’s an example combining past and future that I would find perfectly acceptable:
Although the dress rehearsal went badly, tonight’s performance will be much better. CORRECT
It’s true that there are so-called authorities who insist on a one-tense-per-sentence policy. But that’s not how the real world works. This is how that example sentence would look if we let them have their way:
Although the dress rehearsal went badly, tonight’s performance would be much better. INCORRECT
“Would” is the past tense of “will” (isn’t language amazing?), so we now have a perfectly consistent sentence. The problem, though, is that nobody would ever say it that way.
Bottom line: Be cautious about mixing tenses – and remember that real-world English usage trumps every rule.