Questions & Answers about Я хочу помыть машину.
What case is машину, and why is it used here?
машину is in the accusative case, which marks the direct object of a transitive verb. Since помыть (“to wash,” perfective) directly acts on the car, машина (nominative) changes to машину (accusative singular feminine).
Why is the perfective verb помыть used instead of the imperfective мыть?
Russian verbs have aspects. The perfective помыть indicates you intend to complete the action (“wash it entirely”). The imperfective мыть stresses the process or a habitual action (“to be washing” or “wash regularly”), which doesn’t fit the idea of wanting to finish washing once.
Can I say Я хочу мыть машину, and if so, what’s the difference?
You could, but it sounds odd. Я хочу мыть машину implies you want to be in the process of washing or do it repeatedly, not necessarily finish it. For a single, completed wash, use Я хочу помыть машину.
Is the pronoun я necessary before хочу, or can it be omitted?
It can be omitted because хочу (want) is conjugated for the first person singular. Хочу помыть машину is perfectly grammatical. Including я adds clarity or emphasis.