In the book " Aristotle and Dante Discover the Secrets of the Universe" the author writes the following sentence.
"That's when I heard his voice, kind of squeaky(...)"
Is it correct to say "that is" in in the same sentence of a verb used in the past tense?
The use of both tenses confuses me, can anyone explain what rule that is?
It is correct and there's no rule in English saying otherwise. Here's an article that will explain it better: https://www.veritasprep.com/blog/2017/08/multiple-tenses-gmat-sentence/. The main takeaway from it is:
The tenses of verbs in a sentence must be consistent when the actions happen at the same time. When dealing with actions that occur at different points in time, however, we can use multiple tenses in the same sentence.
Keeping that in mind, all of the following are correct:
You do have a point there. It could be both ways, depending on the meaning.
In the given example, it would rather be "That was when I heard his voice, ..."
However, that's (but not that is) is idiomatic and is frequently used both in the present as well as the past tense. So you will see such expressions as "That's when I heard his voice, … "
On the other hand, "That's what I had told him" uses a mixed tense and is is the correct tense because the fact still holds true. Whereas "That was what I had told him" could imply something has changed with the fact now.
HTH.
