اخي سريع بالكتابة، بس انا ابطا منه لما بكون تعبان.

Because in Arabic, the verb to be is usually left out in the present tense.

So:

اخي سريع بالكتابة
literally looks like my brother fast at-writing

but it naturally means:

My brother is fast at writing.

This is completely normal in both Levantine and Standard Arabic. If you want was or will be, then Arabic usually does use a verb.

For example:

اخي كان سريع بالكتابة = My brother was fast at writing
اخي رح يكون سريع بالكتابة = My brother will be fast at writing

اخي means my brother.

It comes from:

أخ = brother
أخي = my brother

In casual Arabic writing, especially online or in dialect, people often leave out the hamza at the beginning, so اخي is just an informal spelling of أخي.

A learner should recognize both:

أخي = more careful/standard spelling
اخي = very common casual spelling

In spoken Levantine, people may also say other forms like أخوي or regional variants, but in this sentence اخي simply means my brother.

سريع means fast.
الكتابة means writing.
So سريع بالكتابة means something like:

fast in writing
or more naturally, fast at writing

The بـ here often gives the sense of in / with / at, depending on context. In expressions like this, it often connects an adjective to an activity or skill.

So:

شاطر بالرياضيات = good at math
ضعيف بالإنجليزي = weak in English
سريع بالكتابة = fast at writing

This is very natural in Levantine.

In Arabic, abstract nouns and activity nouns often appear with الـ in expressions like this.

So بالكتابة here means something like:

in writing
when it comes to writing
with writing

Using الـ sounds natural and idiomatic. It is not necessarily making it the writing in the way English would.

So do not try to translate it word-for-word too mechanically.
سريع بالكتابة is just a natural way to say fast at writing.

بس is a very common Levantine word. Here it means but.

So:

اخي سريع بالكتابة، بس انا ابطا منه...
= My brother is fast at writing, but I’m slower than him...

Important note: بس can also mean only / just in other contexts.

For example:

بس هيك = that’s all / just that
بس بدي اسأل = I just want to ask

But in your sentence, it clearly means but.

This is the Arabic comparative structure.

ابطا = slower
منه = than him

So:

انا ابطا منه = I am slower than him

The word ابطا corresponds to Standard Arabic أبطأ. In casual writing, the hamza is often dropped, so you may see:

أبطأ = more careful spelling
ابطا = casual spelling

The pattern is very common in Arabic:

أكبر منه = bigger than him
أصغر مني = smaller than me
أسرع منك = faster than you
أبطا منه = slower than him

That is a great question. Yes, من often means from, but after a comparative adjective it means than.

So in Arabic:

أكبر من = bigger than
أحسن من = better than
أبطأ من = slower than

Then you attach the pronoun:

مني = than me
منك = than you
منه = than him
منها = than her

In Levantine pronunciation, منه is often said as minno.

So:

ابطا منه
is understood as
slower than him, not slower from him.

It means when I’m tired.

Breakdown:

لما = when
بكون = I am / I’m being / I happen to be
تعبان = tired

So the whole phrase means:

when I’m tired

In natural English, that is the best translation.

A slightly more literal sense is:

when I am in a tired state

But you should learn it simply as when I’m tired.

In Levantine Arabic, the prefix بـ is very common with the present tense.

So:

بكون = I am / I become / I’m usually
depending on context

In this sentence, لما بكون تعبان means when I’m tired.

This is a normal Levantine way to talk about a present or recurring situation. It does not sound strange here.

Compare:

لما بكون مشغول = when I’m busy
لما بكون نعسان = when I’m sleepy
لما بكون تعبان = when I’m tired

So the بـ is part of the normal dialectal present-tense form.

Because the speaker here is using the masculine singular form.

تعبان = tired, said by a male
تعبانة = tired, said by a female

So if a woman said the same sentence, she would usually say:

اخي سريع بالكتابة، بس انا ابطا منه لما بكون تعبانة.

Everything else can stay the same; only the adjective describing the speaker changes.

It is best described as Levantine colloquial writing, with some features that are also understandable to speakers of Standard Arabic.

Signs that it is Levantine include:

بس for but
بكون as a dialect present form
casual spellings like اخي and ابطا

At the same time, many of the words are close enough to Standard Arabic that a learner may recognize them easily.

So this is a good example of how written dialect often looks:
mostly Arabic vocabulary you may know, but with colloquial grammar and spelling.

A rough pronunciation would be:

akhi sarii3 bil-kitaabe, bas ana abta minno lamma bkuun ta3baan

A few notes:

• سريع is often pronounced roughly sarii3
• منه is commonly pronounced minno
• بكون sounds like bkuun
• تعبان sounds like ta3baan
• ع is often written as 3 in transliteration

Also, depending on region and speaking style, pronunciation can vary a little, but this gives you a good Levantine-style approximation.

Yes. Levantine often allows small variations while keeping the same meaning.

For example, people might say things like:

أخي سريع بالكتابة، بس أنا أبطأ منه لما بكون تعبان.
This is just a more careful spelling.

Or some speakers might prefer another way to say my brother, depending on region.

But the structure of your sentence is already natural and understandable:

• statement about your brother
• بس for contrast
• comparative with منه
• time/state phrase with لما بكون تعبان

So as a learner, this sentence is a very useful model.