Breakdown of Achetez de l'engrais pour que nous semions les graines aujourd'hui.
nous
we / us (object pronoun)
de
of / from / any (in negative sentences) / about (concerning)
acheter
to buy
aujourd'hui
today
Questions & Answers about Achetez de l'engrais pour que nous semions les graines aujourd'hui.
Why is semions used instead of semons?
The phrase pour que means "so that" or "in order that," and it is a conjunction of purpose that always requires the subjunctive mood. Because of this rule, we must use the subjunctive form nous semions instead of the regular present tense nous semons.
semions looks exactly like the imperfect tense. How do I know it is the subjunctive here?
The context tells you! Because it follows pour que, it has to be the subjunctive. However, you are right about the spelling: for the nous and vous forms of almost all verbs, the present subjunctive is perfectly identical to the imperfect tense (l'imparfait).
Could I say pour semer instead of pour que nous semions?
Not in this sentence! You use pour + the infinitive (like pour semer) only when the subject is the same for both actions. Here, the subjects are different: "you" are buying the fertilizer, but "we" (nous) are sowing the seeds. When there is a subject change like this, you must use pour que + the subjunctive.
Why does it say de l'engrais rather than just l'engrais?