Learn French: L'astronaute qui voyagera dans l'espace sera prêt. - The astronaut who will travel in space will be ready.

The verb être (to be) is highly irregular. Instead of adding endings to its infinitive, it uses the special future stem ser-. Because the subject is l'astronaute (equivalent to il or elle), we add the standard -a ending to get sera (will be).

Both verbs are in the future tense, so why does voyagera include its whole infinitive while sera doesn't?

This shows the difference between regular and irregular future stems. voyagera is regular, so you take the full infinitive voyager and add the future ending -a. But sera comes from être, which is irregular. You must memorize its special stem (ser-) and attach the -a ending directly to that.

How do we know if the astronaut in this sentence is a man or a woman?

We look at the adjective at the end of the sentence! The noun astronaute is spelled the exact same way for a man or a woman (un astronaute or une astronaute). However, the adjective is the masculine prêt (ready). If it were a female astronaut, the sentence would end with the feminine form, prête.

Why do we use qui here instead of que?