Hello, dear student! One always gives much consideration to verbs as most students and foreigners constantly complain about mistakes and misunderstanding. A verb, is really such a difficult part of speech, which requires a lot of efforts, experience and knowledge - how, where and with what we can use them in speech. Before we start learning certain verbs in this article, I recommend you to begin with reading and recalling aspects and tenses of verbs, then you will understand this topic much clearer.
Well, today we will give consideration to the verb "спать" [spat'] - to sleep. Like any other verbs, it is changed in tenses and aspects:
Я сплю - Мы спим [Ya splyu - My spim] - I sleep, we sleep
Ты спишь - Вы спите [Ty spish' - My spim] - You sleep - You (plural) sleep
Он, она спит - Они спят [On, ana spit - Ani spyat] - He, she sleeps - They sleep
Она спит крепко по ночам [Ana spit krepka pa nacham] - She sleeps deeply at night
Perfective aspect: there is no perfective aspect in present tense!
Я, ты, он спал [Ya, ty, on spal] - I, you, he was sleeping
Я, ты, она спала [ya, ty, ana spala] - I, you, she was sleeping
Мы, вы, они спали [My, vy, ani spali] - We, you (plural), they were sleeping
In this case we cannot use it! But we can say, that "человек хорошо поспал или выспался" - a person slept well or had his sleep out.
Я буду спать [ya budu spat'] - I will sleep
Ты будешь спать [Ty budish spat'] - You will sleep
Он, она будет спать [On, ana budit spat'] - He, she will sleep
Мы будем спать [My budim spat'] - We will sleep
Вы будете спать [Vy budite spat'] - You will sleep
Они будут спать [Ani budut spat'] - They will sleep
In this case we also do not use it like in the past tense. But we can say that "человек поспит или хочет/желает выспаться (на выходных)" - a person will sleep or wants to sleep enough (on weekend).
There is also the imperative mood of verbs and this verb will be as follows:
Доброй ночи! Спи крепко и встань пораньше! [Dobraj nochi! Spi krepka i fstan' paran'ishe] - Good night! Have a deep sleep and get up early!
The verb "спи" [spi] is imperfective. You can also see the following form in colloquial speech: "Выспись" or "Поспи немного" - Sleep enough or Get some sleep.
And finally, I'd like to draw your attention to the writing of this verb in the future tense: do not use the form "будИшь" спать - it is also a major mistake!