Learn French: Les astronautes pourront bientôt marcher sur Mars. - The astronauts will soon be able to walk on Mars.

It is the futur simple (future tense) of the verb pouvoir (to be able to / can). Pouvoir has a highly irregular future stem: pourr-. Since the subject is les astronautes (they / ils), we add the standard -ont ending to get pourront.

How do I pronounce the double 'r' in pourront?

In French, the double 'r' in the future tense of verbs like pouvoir (pourr-) is pronounced a bit stronger and slightly longer than a single 'r' in the throat. This slightly extended sound helps native speakers clearly hear that you are using the future tense.

Why is bientôt placed in the middle of the verbs?

In French, short and common adverbs like bientôt (soon) are usually placed immediately after the conjugated verb (pourront) and before the infinitive verb (marcher). English often puts 'soon' at the very end of the sentence, but French prefers it tucked right after the conjugated verb.

Why doesn't marcher have a future tense ending too?