المتجر الذي في هذا الشارع مغلق اليوم، لذلك لا أشتري القميص.

It has two parts:

1. المتجر الذي في هذا الشارع مغلق اليوم
   This is basically a nominal sentence in Arabic:

   • المتجر = the topic/subject
   • مغلق = the predicate (closed)
   • الذي في هذا الشارع = a relative clause describing المتجر

2. لذلك لا أشتري القميص
   This is a verbal sentence:

   • لا أشتري = I do not buy / I am not buying
   • القميص = the shirt

So the full sentence is: a statement about the store, followed by a consequence.

الذي means that / which / who as a relative pronoun.

In this sentence:

• المتجر الذي في هذا الشارع
• literally: the store that is in this street

It connects المتجر to the description that follows it.

Also, الذي is the masculine singular form, which matches المتجر because متجر is masculine singular.

Useful comparison:

• الذي = masculine singular
• التي = feminine singular
• الذين = masculine plural
• اللاتي / اللواتي = feminine plural

Because الذي introduces a relative clause: the store that is in this street.

Without الذي, المتجر في هذا الشارع would usually just mean something more like:

• The store is in this street

That is a complete sentence, not a noun plus a defining clause.

So:

• المتجر الذي في هذا الشارع = the store that is in this street
• المتجر في هذا الشارع = the store is in this street

That is an important difference.

In Arabic, the verb to be is usually not expressed in the present tense.

So Arabic says:

• المتجر مغلق
• literally: the store closed
• natural English: the store is closed

The same thing happens inside the relative clause:

• الذي في هذا الشارع
• literally: that in this street
• natural English: that is in this street

If you want past or future, Arabic usually uses forms of كان:

• كان المتجر مغلقًا = the store was closed
• سيكون المتجر مغلقًا = the store will be closed

In standard Arabic, when a demonstrative like هذا comes before a noun, that noun is usually definite, so it normally takes الـ.

So:

• هذا الشارع = this street
• literally: this the-street

That may feel strange to an English speaker, but it is the normal Arabic pattern.

Other examples:

• هذا البيت = this house
• هذه السيارة = this car

مغلق is not a finite verb here. It is a passive participle used like an adjective/predicate.

It comes from the verb أغلق = to close.

So:

• مغلق = closed

In this sentence, it functions as the predicate of المتجر:

• المتجر مغلق = the store is closed

So a good way to think of it is:

• form: adjective/passive participle
• function in the sentence: predicate

لذلك means therefore, so, or for that reason.

It is made of:

• لِـ = for / to
• ذلك = that

So literally it is something like for that or for that reason.

In this sentence it links the two ideas:

• the store is closed today
• therefore I am not buying the shirt

It is a common connector in formal Arabic.

Good question. These are related but not identical:

• لا أشتري = I do not buy / I am not buying
• لن أشتري = I will not buy

So:

• لا negates the present/imperfect
• لن negates the future

In this sentence, لا أشتري القميص can naturally mean I’m not buying the shirt as a present decision/result.

If you want to make the future meaning more explicit, you could say:

• لذلك لن أشتري القميص = therefore I will not buy the shirt

So the original sentence is understandable, but لن أشتري would sound more explicitly future-oriented.

Because of the prefix أـ on the imperfect verb.

The verb is from اشترى = to buy.

Its imperfect forms include:

• أشتري = I buy
• تشتري = you buy / she buys
• يشتري = he buys
• نشتري = we buy

So the أـ tells you it is first person singular: I.

Also, this verb is a little tricky because it ends in a weak letter in the past form:

• اشترى (past)
• يشتري / أشتري (imperfect)

Because القميص means the shirt, referring to a specific shirt.

If you said:

• أشتري القميص = I buy / am buying the shirt
• أشتري قميصًا = I buy / am buying a shirt

So the presence of الـ shows definiteness.

A learner may notice that English sometimes says a shirt in similar contexts, but Arabic here chooses the shirt, probably because the shirt is already known or specific in the situation.

Fully vocalized, it would be:

المتجرُ الذي في هذا الشارعِ مغلقٌ اليومَ، لذلك لا أشتري القميصَ.

Here is what the endings show:

• المتجرُ: nominative, because it is the subject/topic
• الشارعِ: genitive, because it comes after في
• مغلقٌ: nominative, because it is the predicate
• اليومَ: accusative as an adverb of time
• القميصَ: accusative, because it is the direct object

In normal Arabic writing, these short vowels are usually omitted, so learners have to infer them from grammar.