يمكن أن نكتب رسالة الآن.

Because يمكن is being used as an impersonal expression meaning something like it is possible or it can be done.

So the structure is:

• يمكن = it is possible / can
• أن نكتب = that we write / for us to write

Literally, the sentence is closer to:

• It is possible that we write a letter now

In natural English, that usually becomes:

• We can write a letter now

So Arabic is not always copying the English structure word-for-word.

يمكن comes from the root related to possibility or ability. In this sentence, it means:

• it is possible
• one can
• it can be done

In context, English often translates it as can.

A useful point:

• يمكن أن نكتب = we can write
• but more literally = it is possible for us to write

So يمكن is not a normal verb with a visible subject like we can in English. It often behaves more like an impersonal phrase.

أن is a particle placed before a present-tense verb. It often means to or that depending on how the sentence is translated.

Here:

• أن نكتب = to write / that we write

After verbs or expressions of possibility, desire, intention, etc., Arabic commonly uses أن + present verb.

So:

• يمكن أن نكتب = it is possible to write / we can write

This is a very common pattern in MSA.

Because نكتب is the first person plural form of the present/imperfect verb:

• أكتب = I write
• نكتب = we write
• يكتب = he writes
• تكتب = she writes / you write

Since the meaning is we can write, the correct form is نكتب.

Also, Arabic usually does not need a separate subject pronoun here, because the verb already shows who is doing the action.

You could say نحن for emphasis, but it is usually not necessary.

So:

• يمكن أن نكتب رسالة الآن = normal
• يمكن أن نحن نكتب رسالة الآن = not the normal way to say it

If you want to use نحن, it would usually appear in a more carefully structured sentence for emphasis, but with this pattern Arabic normally just uses the verb form نكتب and leaves the pronoun unstated.

So the natural MSA sentence is simply:

• يمكن أن نكتب رسالة الآن

There actually is a grammatical change, but in normal unvoweled writing you usually do not see it.

After أن, the present verb goes into the subjunctive.

Fully vowelled:

• نكتبُ = indicative
• نكتبَ = subjunctive after أن

So in a fully vocalized text, the last vowel changes from u to a.

But since everyday Arabic writing usually leaves out short vowels, both forms look the same:

• نكتب

That is why learners often do not notice the change at first.

رسالة without الـ means a letter, not the letter.

So:

• رسالة = a letter
• الرسالة = the letter

Since the English meaning is general and not referring to a specific known letter, the indefinite form is used.

If you meant a specific letter already known in the conversation, you could say:

• يمكن أن نكتب الرسالة الآن = We can write the letter now

It is the direct object of نكتب, so it is in the accusative case.

In fully vowelled Arabic, that would be:

• رسالةً

So the fully vocalized sentence would be:

• يُمْكِنُ أَنْ نَكْتُبَ رِسَالَةً الآنَ

In normal writing, the case ending is usually omitted, so you just see:

• رسالة

الآن means now. It is an adverb of time.

It tells you when the action can happen:

• يمكن أن نكتب رسالة الآن = We can write a letter now

It is very common to place time expressions like this near the end of the sentence, although Arabic word order can be somewhat flexible.

Yes, Arabic allows some flexibility, though some orders sound more natural than others.

For example:

• يمكن أن نكتب رسالة الآن = very natural
• الآن يمكن أن نكتب رسالة = Now we can write a letter
• يمكن الآن أن نكتب رسالة = also possible

The original sentence is simple and natural MSA. Moving الآن changes emphasis more than basic meaning.

They are similar, but not always identical in tone.

• يمكن أن نكتب = it is possible / we can
• نستطيع أن نكتب = we are able to / we can

Very roughly:

• يمكن often emphasizes possibility
• نستطيع often emphasizes ability

In many everyday contexts, both can translate as can.

For example:

• يمكن أن نكتب رسالة الآن = We can write a letter now
• نستطيع أن نكتب رسالة الآن = We are able to write a letter now / We can write a letter now

With a following present-tense verb, أن is normally used:

• يمكن أن نكتب = correct and standard

Leaving it out would not be standard MSA in this structure.

However, Arabic also has other ways to express a similar idea, such as:

• يمكننا كتابة رسالة الآن = We can write a letter now
• literally: It is possible for us to write a letter now

So أن is not always required in every possible rewording, but in يمكن + present verb, it is normally there.

A careful full-vowel pronunciation would be:

• yumkinu ʾan naktuba risālatan al-ʾāna

Word by word:

• يُمْكِنُ = yumkinu
• أَنْ = ʾan
• نَكْتُبَ = naktuba
• رِسَالَةً = risālatan
• الآنَ = al-ʾāna

In normal speech or pause, the final case endings are often reduced or not pronounced fully, especially outside very formal recitation.

Yes. It is grammatical and natural in MSA.

It is a straightforward sentence using a very common pattern:

• يمكن أن + present verb

So this is a good model sentence for learning how to say can or it is possible to in Modern Standard Arabic.