تعديلات الملابس مهمة.

A simple breakdown is:

• تعديلات = alterations / modifications
• الملابس = the clothes / clothing
• مهمة = important

So the phrase as a whole means something like clothing alterations are important or alterations to clothes are important.

Because Arabic often leaves out the present-tense verb to be.

So in English you say:

• Clothing alterations are important

But in Arabic, in the present tense, you usually just put:

• the subject: تعديلات الملابس
• the predicate: مهمة

This is called a nominal sentence.
If you wanted a past or future meaning, Arabic would use a verb, but in the present tense it is normally omitted.

It is an iḍāfa structure, often called a construct phrase.

That means:

• تعديلات = the first noun
• الملابس = the second noun, which specifies what kind of alterations

So تعديلات الملابس literally means something like:

• alterations of the clothes
• more naturally in English: clothing alterations or alterations to clothes

This structure is extremely common in Arabic.

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

So Arabic says:

• تعديلات الملابس

not:

• التعديلات الملابس

Also, the first noun in an iḍāfa does not take tanwīn either. In fully vowelled Arabic, you would have:

• تعديلاتُ الملابسِ

not:

• تعديلاتٌ الملابسِ

That is a standard rule of the construct phrase.

Because it is the second noun in an iḍāfa.

In Arabic, the second part of an iḍāfa is automatically genitive. So with full case endings, it is:

• الملابسِ

This is true even if English would not show any special ending.

So:

• تعديلاتُ = nominative, because it is the subject
• الملابسِ = genitive, because it is the second part of the iḍāfa
• مهمةٌ = nominative, because it is the predicate

This is a very common Arabic pattern.

تعديلات is a non-human plural, and in Modern Standard Arabic, non-human plurals usually take feminine singular agreement in adjectives and predicates.

So Arabic says:

• تعديلات الملابس مهمة

Literally, the grammar treats the plural noun a bit like a feminine singular for agreement purposes.

Compare the same pattern:

• السيارات جديدة = The cars are new
• الكتب مفيدة = The books are useful

Even though the nouns are plural, the adjective is feminine singular.

Yes. تعديلات is a plural in -ات, often called the sound feminine plural form.

But that does not mean the singular noun has to be feminine in meaning.

Arabic often uses -ات to make plurals of:

• abstract nouns
• verbal nouns
• many non-human nouns
• some borrowed words

So:

• تعديل = alteration / modification
• تعديلات = alterations

This is normal and does not mean the word refers to females.

A fully vowelled version is:

• تَعْدِيلَاتُ الْمَلَابِسِ مُهِمَّةٌ

A careful pronunciation is roughly:

• taʿdīlātu al-malābisi muhimmatun

In pause, many speakers would say something closer to:

• taʿdīlāt al-malābis muhimmah

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

Grammatically, تعديلات الملابس is a definite phrase, because the second noun الملابس is definite.

So literally, it can correspond to:

• the clothing alterations
• the alterations of the clothes

But in natural English, Arabic definite phrases are often translated more smoothly as a general category, especially in short statements like this. So:

• clothing alterations are important

is usually the best translation.

The exact English wording depends on context.

Yes, but تعديلات الملابس مهمة is the most neutral and straightforward order.

This is:

• subject: تعديلات الملابس
• predicate: مهمة

Arabic can sometimes put the predicate first for style, emphasis, or context, but for a learner, the given order is the clearest basic pattern.

So if you are producing simple Modern Standard Arabic, this sentence is a very good model to follow.