سياسة الاستبدال والاسترجاع مهمة.

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

So:

• سياسة الاستبدال والاسترجاع مهمة
  literally looks like: The exchange-and-return policy important
• but it means: The exchange and return policy is important

If you wanted the past tense, Arabic would use كان or one of its forms, for example:

• كانت سياسة الاستبدال والاسترجاع مهمة
  The exchange and return policy was important

Yes. It is a single noun phrase meaning the exchange and return policy.

It is built as an iḍāfa construction, often called a genitive/possessive construction in English grammar discussions.

Breakdown:

• سياسة = policy
• الاستبدال = exchange / replacement
• الاسترجاع = return / refund / taking back

So the structure is:

• سياسةُ الاستبدالِ والاسترجاعِ

This literally has the sense of:

• policy of exchange and return

In natural English, that becomes the exchange and return policy.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So Arabic says:

• سياسة الاستبدال والاسترجاع

not:

• السياسة الاستبدال والاسترجاع ❌

Even though سياسة itself has no الـ, the whole phrase becomes definite because the following noun is definite:

• الاستبدال = the exchange/replacement
• الاسترجاع = the return

So the whole phrase is understood as definite: the exchange and return policy.

Because it describes سياسة, and سياسة is a feminine noun.

In Arabic, adjectives must agree with the noun they describe in:

• gender
• number
• definiteness

So:

• سياسة = feminine singular
• مهمة = feminine singular

If the noun were masculine, you would use مهم instead.

Compare:

• الكتاب مهم = The book is important
• السياسة مهمة = The policy is important

That is the normal pattern in Arabic.

Arabic usually puts the adjective after the noun it describes.

So:

• سياسة مهمة = an important policy
• السياسة مهمة = the policy is important

In your sentence, the full subject comes first:

• سياسة الاستبدال والاسترجاع = the exchange and return policy

Then the predicate/adjective comes after it:

• مهمة = important

Because here مهمة is the predicate of the sentence, not an adjective inside the noun phrase.

Compare these two ideas:

1. سياسة الاستبدال والاسترجاع مهمة
   = The exchange and return policy is important
   Here مهمة is the statement about the subject.

2. سياسة الاستبدال والاسترجاع المهمة
   = the important exchange and return policy
   Here المهمة is an adjective modifying the noun inside one noun phrase.

So in your sentence, مهمة is indefinite because it is functioning like important in is important, not like the important.

Because both nouns are definite and linked by و (and).

So Arabic is literally saying:

• the exchange and the return

This is very normal. When two nouns are coordinated with و, each one often keeps its own الـ:

• البيع والشراء = buying and selling
• المدرس والطالب = the teacher and the student

So:

• الاستبدال والاسترجاع = the exchange and the return

They are both verbal nouns.

• الاستبدال comes from a verb meaning to replace / to exchange
• الاسترجاع comes from a verb meaning to get back / recover / return

In store or customer-service language:

• الاستبدال often means exchange or replacement
• الاسترجاع often means return and sometimes carries the sense of refund/returning an item

So together they are a standard retail phrase for exchange and return.

It is a nominal sentence.

A nominal sentence in Arabic begins with a noun or noun phrase, not a verb.

Here:

• سياسة الاستبدال والاسترجاع = subject/topic
• مهمة = predicate/comment

So the structure is:

• subject + predicate

This is one of the most common sentence types in Arabic.

A fully vowelled version would typically be:

• سِياسَةُ الِاسْتِبْدالِ وَالِاسْتِرْجاعِ مُهِمَّةٌ

Here is why:

• سِياسَةُ ends in ـُ because it is the subject of the nominal sentence
• الاستبدالِ ends in ـِ because it is the second part of the iḍāfa
• الاسترجاعِ also ends in ـِ for the same reason
• مهمةٌ ends in ـٌ because it is the indefinite predicate

In normal everyday writing, these short vowels are usually not written.

A simple pronunciation guide:

• سياسة ≈ si-yaa-sa
• مهمة ≈ mu-him-ma

A few notes:

• سياسة has a long yaa sound in the middle.
• مهمة has a doubled m sound because of the shadda on م: مهمّة
• The final ة in مهمة is pronounced like -a when pausing at the end.

So the whole sentence is approximately:

• siyaasat al-istibdaal wa-l-istirjaaʿ muhimma

That final ة is called tāʾ marbūṭa.

Its pronunciation depends on context:

• when pausing, it usually sounds like -a
• when followed by another word in connected speech or grammar, it can sound like -t

So:

• سياسة by itself: siyaasa
• in connection, as in سياسةُ الاستبدال: it is effectively siyaasatu...

Similarly:

• مهمة at the end of the sentence is usually pronounced muhimma
• if it were followed by something else in fully connected speech, the t could reappear grammatically

Yes. و is the Arabic conjunction and.

In writing, it is attached directly to the following word:

• والاسترجاع = and the return

This is completely normal in Arabic spelling.

So:

• الاستبدال والاسترجاع means:
• exchange and return

Yes, depending on context.

الاستبدال can mean:

• replacement
• substitution
• exchange

In retail language, English often says:

• exchange and return policy
• replacement and return policy

Both can work, but exchange and return policy is often the most natural general translation.

In this sentence, the subject is the full noun phrase:

• سياسة الاستبدال والاسترجاع

and the predicate is:

• مهمة

A good test is this:

Ask, What is important?

Answer:

• سياسة الاستبدال والاسترجاع
  The exchange and return policy

So that whole phrase is the subject.

Then مهمة tells us something about it.

Yes, that is also possible, and you may see similar variations.

For example:

• سياسة الإرجاع والاستبدال مهمة
• سياسة الاستبدال والإرجاع مهمة

The exact wording depends on style and context.

A few notes:

• الإرجاع is another common word for returning
• الاسترجاع can also be used in formal contexts
• stores and websites may choose one standard expression over another

So your original sentence is correct, but it is not the only possible way to express the idea.

Because سياسة is singular.

The sentence is talking about one policy that covers two things:

• exchange
• return

So Arabic treats it as:

• one policy related to exchange and return

That is why مهمة is also singular feminine.

If it were plural, the wording would be different.