Learn Italian: Penso che Marco pianti i semi oggi. - I think Marco is planting the seeds today.

The phrase Penso che (I think that) expresses an opinion or personal thought. In Italian, expressing opinions triggers the subjunctive mood in the following clause. Since piantare (to plant) is a regular -are verb, its present subjunctive form for Marco (he/lui) changes the ending to -i, making it pianti.

Doesn't pianti usually mean you plant in the regular present tense?

Yes, it does! However, in the present subjunctive, the io, tu, and lui/lei forms for regular -are verbs all end in -i. Because these three forms look exactly the same, you usually need a subject pronoun (like io or tu) or a specific subject (like Marco) in the sentence to clarify who is doing the action.

The English translation says is planting. Why aren't we using a form of the verb stare plus the gerund?

You could use the subjunctive continuous form (stia piantando) to heavily emphasize that he is in the exact middle of the action right now. But Italian often uses simple tenses where English relies on continuous ones. The single word pianti translates perfectly well to both plants and is planting.

Why is the word for the seeds i semi? Is the singular word feminine?