خطاب التوصية، مثل الرسالة، واضح.

This is because خطاب التوصية is an iḍāfa construction, often called a genitive or possessive construction.

• خطاب = letter / formal document / address
• التوصية = the recommendation

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun can make the whole phrase definite

So:

• خطاب توصية = a letter of recommendation
• خطاب التوصية = the letter of recommendation

Even though خطاب has no الـ, the whole phrase is definite because the second noun is definite.

Because that is not how iḍāfa works.

In a normal iḍāfa:

• the first noun is left bare
• the second noun shows definiteness if needed

So the correct pattern is:

• خطاب التوصية not
• الخطاب التوصية

The first noun in iḍāfa also does not take tanwīn, and it is directly linked to the second noun.

Not exactly. They overlap, but they are not always identical.

• خطاب often suggests something more formal, such as an address, official communication, or formal letter.
• رسالة is more general and can mean letter or message.

In the fixed expression خطاب التوصية, Arabic commonly uses خطاب for letter of recommendation or reference letter. So here it is a standard collocation, not just any random choice.

Arabic often expresses X of Y using iḍāfa:

• خطاب التوصية = letter of recommendation
• literally: letter + the recommendation

This is the normal Arabic way to say the recommendation letter or the letter of recommendation.

English often uses:

• adjective + noun, or
• noun + of

Arabic often uses:

• noun + noun in iḍāfa

So the order is normal for Arabic.

Because it agrees with خطاب, which is the main noun being described or predicated about.

• خطاب is grammatically masculine singular
• so the predicate adjective is واضح = clear

If the main noun were feminine, you would expect واضحة instead.

For example:

• الرسالة واضحة = the letter is clear
• خطاب التوصية واضح = the recommendation letter is clear

Even though التوصية is feminine, the head noun is still خطاب, so the agreement follows خطاب, not التوصية.

Because in this sentence واضح is the predicate of a nominal sentence, not an attributive adjective inside the noun phrase.

The structure is basically:

• خطاب التوصية = subject
• واضح = predicate

So this means:

• The recommendation letter is clear

In Arabic, predicates are often indefinite, even when the subject is definite.

Compare:

• الكتاب مفيد = the book is useful
• الرسالة طويلة = the letter is long

If واضح were inside the noun phrase as a regular adjective, the structure would be different. But here it is the thing being said about the subject.

It is a nominal sentence.

There is no verb like is written. Arabic often omits the present-tense to be in simple statements.

So:

• خطاب التوصية واضح literally: the recommendation letter clear meaning: The recommendation letter is clear

This is one of the most important patterns in Arabic:

• definite noun phrase + predicate

It is a parenthetical comparison meaning something like:

• like the letter
• as with the letter

So the sentence means that the recommendation letter is clear, and the phrase مثل الرسالة adds a comparison to الرسالة.

Because it is parenthetical, it is set off by commas:

• خطاب التوصية، مثل الرسالة، واضح.

It does not change the basic core of the sentence:

• خطاب التوصية واضح = The recommendation letter is clear.

The middle phrase just adds extra information.

Yes, often it could.

For example, a similar idea could be expressed with:

• خطاب التوصية، كالرسالة، واضح

The prefix كـ also means like / as.

Very roughly:

• مثل = like
• كـ = like / as

Both are common, though the exact style and nuance can vary by context. In many learner-level situations, you can think of them as near equivalents.

The commas mark مثل الرسالة as an inserted parenthetical phrase.

Arabic punctuation is not always used exactly like English punctuation, but here the commas help show that:

• the main statement is خطاب التوصية واضح
• مثل الرسالة is extra, non-core information

Also note that Arabic uses the Arabic comma:

• ، not the English comma:
• ,

A likely fully vocalized version is:

• خِطابُ التَّوصِيَةِ، مِثلَ الرِّسالةِ، واضِحٌ

Here:

• خِطابُ is the subject, so it is nominative
• التوصيةِ is the second term of the iḍāfa, so it is genitive
• واضحٌ is the predicate, so it is nominative

The middle phrase مثل الرسالة can be analyzed in slightly different ways depending on how exactly the comparison is understood, and in normal unvowelled Arabic learners usually do not need to worry too much about that at first. The key point is that the written sentence without vowels is completely normal.

Yes, رسالة is feminine.

You can tell from both meaning and form:

• it ends in ة
• it normally takes feminine agreement

For example:

• الرسالة واضحة = the letter is clear

In this sentence, though, الرسالة is inside the comparison phrase مثل الرسالة, so it does not control the agreement of واضح. The predicate agrees with خطاب التوصية, whose head noun is خطاب.

A good learner-friendly breakdown is:

• خطاب التوصية = the recommendation letter
• مثل الرسالة = like the letter
• واضح = clear

So the sentence structure is:

• Subject: خطاب التوصية
• Parenthetical comparison: مثل الرسالة
• Predicate: واضح

That is why the whole sentence means:

• The recommendation letter, like the letter, is clear.