مع أن القميص جميل، لن أشتريه لأن القميص الآخر أرخص.

مع أن means although / even though.

It introduces a contrast:

• مع أن القميص جميل = Although the shirt is جميل

A literal word-for-word translation would not sound natural in English, so it is best learned as a fixed expression meaning although.

In more formal grammar terms, it introduces a subordinate clause of concession.

In present-tense Arabic, the verb to be is usually omitted.

So:

• القميص جميل literally looks like the shirt beautiful
• القميص الآخر أرخص literally looks like the other shirt cheaper

But in normal English, we translate them as:

• The shirt is beautiful
• The other shirt is cheaper

This is very common in Arabic nominal sentences.

لن is used to negate the future, so it means will not or am not going to.

So:

• أشتريه = I buy it / I am buying it / I will buy it depending on context
• لن أشتريه = I will not buy it

A useful basic rule:

• لا often negates present/habitual meaning
• لن negates future meaning

Because Arabic often attaches object pronouns directly to the verb.

Here:

• أشتري = I buy
• ـه = him / it for a masculine singular noun

So:

• أشتريه = I buy it

The ـه refers back to القميص, which is a masculine singular noun.

Because the sentence is contrasting one shirt with the other shirt.

• First: القميص = the shirt
• Later: القميص الآخر = the other shirt

If Arabic used only a pronoun there, it could sound less clear, because we are no longer talking about the same shirt alone; we are introducing another one for comparison.

Arabic also often repeats nouns where English might prefer to avoid repetition.

الآخر means the other here.

So:

• القميص الآخر = the other shirt

It comes after the noun because adjectives normally follow the noun in Arabic.

Also, the adjective agrees with the noun in definiteness:

• القميص = definite
• الآخر = definite

Compare:

• قميص آخر = another shirt / a different shirt
• القميص الآخر = the other shirt

Because القميص is a masculine singular noun, and the adjective must agree with it.

So:

• قميص = masculine singular
• جميل = masculine singular adjective

If the noun were feminine, the adjective would usually be feminine too:

• البلوزة جميلة = The blouse is beautiful

أرخص is on a common Arabic pattern that can express comparative or superlative depending on context.

Here it means cheaper because the sentence is comparing two shirts:

• this shirt is beautiful
• the other shirt is cheaper

So:

• أرخص = cheaper

In other contexts, the same pattern can also mean something like cheapest, but context tells you which meaning is intended.

لأن means because.

So:

• لأن القميص الآخر أرخص = because the other shirt is cheaper

The difference is:

• مع أن introduces a contrast: although
• لأن introduces a reason: because

So the sentence has this structure:

• Although the shirt is beautiful,
• I will not buy it
• because the other shirt is cheaper.

Yes, in full grammatical analysis they do.

Both contain أنّ in this kind of sentence, and أنّ makes the following noun formally accusative.

So in a fully vowelled version, you would see forms like:

• مع أنَّ القميصَ جميلٌ
• لأنَّ القميصَ الآخرَ أرخصُ

But in normal unvowelled writing, those case endings are usually not shown, so you just see:

• مع أن القميص جميل
• لأن القميص الآخر أرخص

A beginner does not need to produce all the case endings right away, but it is useful to know why they exist.

Yes, but the structure would change slightly.

For example:

• القميص جميل، لكنني لن أشتريه لأن القميص الآخر أرخص.

This means:

• The shirt is beautiful, but I will not buy it because the other shirt is cheaper.

That is very close in meaning.

The version with مع أن feels more like:

• Although the shirt is beautiful...

The version with لكن feels more like:

• The shirt is beautiful, but...

Both are natural, but they are not exactly the same structure.