Learn Italian: Lavora maggiormente la mattina. - He works mostly in the morning.

It comes from the adjective maggiore (greater). The rule is that when an adjective ends in -le or -re, you must drop the final -e before adding the adverb suffix -mente. So, maggiore becomes maggior + mente = maggiormente.

Can I use soprattutto instead of maggiormente in this context?

Yes, soprattutto (mostly) or principalmente (mainly) would work perfectly here. Maggiormente means 'to a greater extent' and is a slightly more formal alternative that elevates your vocabulary.

Why does the sentence use la mattina to say 'in the morning' instead of using a preposition?

To talk about a habitual action happening at a specific time of day, Italian typically uses just the definite article, like la mattina or il pomeriggio. You can also use the preposition di (di mattina), which means the exact same thing here.

Could this sentence also mean 'She works mostly in the morning' or 'You work mostly in the morning'?