Learn Finnish: Vahdin laukkua, jos sinä ostat kahvia. - I will watch the bag if you buy coffee.

The verb vahtia (to watch or keep an eye on) is a partitive verb. This means its object must always take the partitive case, even if you are watching the entire, complete bag.

Why is kahvia also in the partitive case?

Here, the partitive is used because kahvi is an uncountable mass noun and the sentence refers to an unspecified amount of it. If you were buying one specific, countable coffee (like a single cup), you would use the accusative case, kahvin.

The English translation says 'I will watch', but the Finnish verb vahdin is in the present tense. How do we express the future here?

Finnish does not have a separate future tense. You simply use the present tense for future actions. The context of the sentence, especially the condition jos (if), makes it completely clear that the action is a plan for the future.

Is it necessary to include the pronoun sinä in jos sinä ostat kahvia?

No, it is optional. The -t ending on the verb ostat already indicates that 'you' are the one buying. Including sinä here just adds a little bit of emphasis.