EXERCISE 28–8 Verbs in negative and conditional sentences
Example:

If we have a choice, we would travel to Italy every year.

If we had a choice, we would travel to Italy every year.

In the second sentence, the past tense is used in the if clause and the modal would is used with the base form, travel, in the independent clause. The first sentence incorrectly mixes present tense and modal plus base form.