لماذا تنامين على الكرسي ولا تنامين على السرير؟

تنامين means the speaker is talking to one female: you (feminine singular) sleep / are sleeping.

In Modern Standard Arabic, verbs often show:

• person: I / you / he / she / we / they
• number: singular / dual / plural
• gender: masculine / feminine

So تنامين specifically identifies the subject as you, feminine singular.

Because Arabic usually does not need an explicit subject pronoun when the verb already shows it.

So:

• تنامين already means you (feminine singular) sleep
• adding أنتِ is possible, but not necessary

You could say:

• لماذا أنتِ تنامين على الكرسي...؟

But that would usually sound more emphatic, as if you are stressing you.

That ending is part of the second-person feminine singular form in the imperfect/present tense.

From the verb نام (to sleep), the relevant forms are:

• أنتَ تنامُ = you (masc. sg.) sleep
• أنتِ تنامين = you (fem. sg.) sleep
• هي تنامُ = she sleeps

So the -ين in تنامين is one of the signals that the verb is addressed to a feminine singular person.

In this sentence, yes, because the verb is in the normal indicative imperfect form: تنامين.

But that final ن is not always kept. In some environments, it drops. For example:

• أنتِ تنامين = you are sleeping
• لن تنامي = you will not sleep
• لا تنامي = do not sleep

So the final ن is present here because this is a regular present-tense statement/question, not a subjunctive, jussive, or negative command form.

Here it means you do not sleep / you are not sleeping, not a command.

That is an important difference.

• لا تنامين = you do not sleep / you are not sleeping
• لا تنامي = don’t sleep (said to one woman)

So in this sentence, ولا تنامين على السرير is a statement inside the question: and you are not sleeping on the bed.

ولا is made of:

• و = and
• لا = not

So here it means something like:

• and not
• while not
• instead of ... not

In natural English, the whole sentence may be translated more smoothly than word-for-word, but grammatically ولا is connecting the two parts and negating the second verb.

It is not the word for or.

Arabic often repeats the verb to make the contrast clear and balanced:

• تنامين على الكرسي
• ولا تنامين على السرير

This repetition is very normal and natural in Arabic.

In English, we often avoid repetition:

• Why are you sleeping on the chair and not on the bed?

But Arabic commonly keeps the full verb in both halves.

Because Arabic normally uses على (on / upon) with both of these nouns.

So:

• على الكرسي = on the chair
• على السرير = on the bed

Even if English sometimes prefers slightly different phrasing depending on context, Arabic comfortably uses على for both here.

الـ is the definite article, meaning the.

So:

• الكرسي = the chair
• السرير = the bed

Arabic often uses the definite article in places where English might also use the, especially when the objects are understood from the situation. The speaker likely means a specific chair and a specific bed known in context.

They are in the genitive case because they come after the preposition على.

In fully vowelled Arabic, you would see:

• على الكرسيِّ
• على السريرِ

In everyday unvowelled writing, those short vowel endings are usually not written, so learners have to know the case from the grammar.

It can be either, depending on context.

The Arabic imperfect verb can cover meanings like:

• do you sleep
• are you sleeping

So لماذا تنامين على الكرسي ولا تنامين على السرير؟ could mean:

• Why do you sleep on the chair and not on the bed? or
• Why are you sleeping on the chair and not on the bed?

Context decides which English translation fits best.

Because لماذا means why, and Arabic question words are usually placed near the beginning of the sentence.

So the structure is very natural:

• لماذا
  • verb + rest of sentence

Also, because لماذا already makes the sentence a question, Arabic does not need a separate yes/no question marker like هل.

You would change تنامين to the masculine singular form تنام:

• لماذا تنام على الكرسي ولا تنام على السرير؟

That means:

• Why are you sleeping on the chair and not on the bed?
  addressed to one male

So the difference is:

• تنامين = you (fem. sg.) sleep
• تنام = you (masc. sg.) sleep

The verb would change to match the group.

Examples:

• to two women:
  لماذا تنامان على الكرسي ولا تنامان على السرير؟

• to a group of women:
  لماذا تنمن على الكرسي ولا تنمن على السرير؟

• to a group of men or a mixed group:
  لماذا تنامون على الكرسي ولا تنامون على السرير؟

Arabic verbs agree with the person being addressed much more explicitly than English verbs do.