Is your tense consistent?

Using a consistent verb tense in your writing is important, because it establishes when the story takes place and how the events relate to each other.

Nevertheless, it is not necessarily true that you can't - or shouldn't - use more than one tense in the same sentence. For example:

I hand the detective a photograph of the man who went missing last year.

The action (handing the photograph) takes place in the present, but it makes reference to something in the past (when the man went missing). The reader is unlikely to be confused by this; in fact, this avoids confusion by clarifying the sequence of events from past to present.

On the other hand:

Nancy perches on a bar stool and orders a margarita. When the drink arrives, she sips it cautiously while the bartender watches. "I'm leaving my husband," she said.

This is confusing. Here is Nancy enjoying a drink in the present, and suddenly we hear about something she said in the past. Did she talk about leaving her husband before or after coming to the bar? Who did she speak to?

Automated grammar checkers do not always find errors like this. A good way to detect confusing content in your writing is to read it aloud or have a critical friend read it aloud to you.