Learn French: Il soumet ce document afin d'expliquer le projet scientifique. - He submits this document in order to explain the scientific project.

Both afin de and pour mean 'in order to' when followed by an infinitive. However, afin de is more formal. It is frequently used in professional, official, or academic contexts, which fits perfectly with someone submitting a document for a scientific project.

Why does it say afin d'expliquer instead of afin de expliquer?

The word de in the phrase afin de must drop its e and add an apostrophe because the following word, expliquer, starts with a vowel. This makes the phrase flow more smoothly when spoken.

Could I use afin qu'il explique instead of afin d'expliquer?

Grammatically yes, but afin d'expliquer is much more natural here. You use afin de + infinitive when the subject of both actions is the same (he is the one submitting, and he is the one explaining). You generally only use afin que + subjunctive when introducing a new subject (for example: he submits it so that WE can explain).

What verb does soumet come from?

It comes from soumettre (to submit). While it is an irregular verb, it follows the exact same conjugation pattern as the very common verb mettre (to put). To form the present tense for il, you drop the -tre ending, leaving il soumet.