اذا كان هذا القميص ارخص من القميص الاخر، سوف اشتريه اليوم.

إذا often introduces a future condition that is seen as realistic or possible. In English, it can be translated as either if or when, depending on context.

In this sentence, if is the best translation: If this shirt is cheaper...

Compared with إن, إذا often sounds more like a real, expected situation, while إن can feel more neutral or more hypothetical.

This is a very common learner question.

Even though كان is the past form of to be, Arabic often uses past-form verbs after إذا to talk about a future condition. So:

• إذا كان هذا القميص أرخص... means If this shirt is cheaper...

It does not mean if this shirt was cheaper in this context.

Also, إذا usually prefers a verbal clause after it, so كان helps turn هذا القميص أرخص into a clause that fits naturally after إذا.

Because قميص is a masculine singular noun.

So the correct demonstrative is:

• هذا = this, for masculine singular
• هذه = this, for feminine singular

So:

• هذا القميص = this shirt

If the noun were feminine, then you would use هذه.

أرخص is the comparative form, meaning cheaper. It follows the common Arabic comparative pattern أفعل.

So:

• رخيص = cheap
• أرخص = cheaper / cheapest, depending on context

When it is followed by من, it clearly has a comparative meaning:

• أرخص من = cheaper than

So:

• أرخص من القميص الآخر = cheaper than the other shirt

In fully vowelled grammar, after كان, it would be أرخصَ because it is the predicate of كان.

In Arabic, من is the normal word used after a comparative adjective to mean than.

Examples:

• أكبر من = bigger than
• أحسن من = better than
• أرخص من = cheaper than

So in this sentence, من is simply the standard way to say than.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• القميص الآخر = the other shirt

Not:

• الآخر القميص

Also, the adjective agrees with the noun in definiteness. Since القميص is definite, الآخر is definite too.

This is the normal Arabic order:

• noun first
• adjective second

Yes, if the context is clear, Arabic can say:

• أرخص من الآخر = cheaper than the other one

But repeating القميص makes the sentence more explicit and clear:

• أرخص من القميص الآخر

That is especially common in careful or formal language, and it is very helpful for learners.

سوف marks the future.

So:

• أشتريه = I buy it / I am buying it
• سوف أشتريه = I will buy it

A shorter future marker is سـ:

• سأشتريه = I will buy it

In many contexts, سوف and سـ mean almost the same thing. Very roughly:

• سوف can sound a bit more formal or emphatic
• سـ is shorter and very common

Both are correct in MSA.

It breaks down like this:

• أشتري = I buy
• ـه = him / it

So:

• أشتريه = I buy it / I will buy it

Here ـه refers to القميص, which is a masculine noun, so ـه is the correct object pronoun.

In careful MSA, yes, many grammar books would prefer:

• إذا كان هذا القميص أرخص من القميص الآخر، فسوف أشتريه اليوم.

That فـ links the condition to the result: if ... then ...

It is especially common when the answer to the condition begins with words like:

• سوف
• سـ
• قد
• لن

So the more textbook-like version is فسوف أشتريه.
Omitting فـ may appear in modern writing, but فسوف is safer in formal MSA.

In careful standard spelling, several words should be written with hamza:

• إذا not اذا
• أرخص not ارخص
• الآخر not الاخر
• أشتريه not اشتريه

In informal typing, many people leave hamzas out, but in formal MSA they should be written.

So a more standard spelling is:

• إذا كان هذا القميص أرخص من القميص الآخر، فسوف أشتريه اليوم.

A fully vowelled version would be:

إِذا كانَ هذا القَميصُ أَرْخَصَ مِنَ القَميصِ الآخَرِ، فَسَوْفَ أَشْتَرِيهِ اليَوْمَ.

A few useful grammar points in that version:

• القميصُ is the subject of كان, so it is nominative
• أرخصَ is the predicate of كان, so it is accusative
• القميصِ الآخرِ is after من, so it is genitive
• اليومَ is an adverb of time, so it is accusative

Arabic word order is fairly flexible.

The sentence as given is natural:

• ... سوف أشتريه اليوم. = ... I will buy it today.

But other placements are also possible, depending on emphasis, for example:

• اليوم سوف أشتريه
• سوف أشتريه اليوم
• سوف أشتري اليوم هذا القميص (with a slightly different structure)

Putting اليوم at the end is a neutral, natural choice.