هذه الأعراض مهمة.

Because الأعراض is a non-human plural noun, and in Modern Standard Arabic, non-human plurals usually take feminine singular agreement.

So even though الأعراض means symptoms and is plural, Arabic treats it grammatically like a feminine singular for agreement purposes in many cases.

That is why you get:

• هذه الأعراض = these symptoms
• not هذا الأعراض

This same pattern also affects the adjective later in the sentence.

For the same reason: الأعراض is a non-human plural, and non-human plurals in MSA normally take feminine singular adjectives.

So:

• الأعراض مهمة = the symptoms are important

Even though the meaning is plural in English, the adjective is feminine singular in Arabic.

This is a very important rule:

• human plural nouns usually take true plural agreement
• non-human plural nouns usually take feminine singular agreement

So مهمة is exactly what you should expect here.

The singular is عَرَض.

So:

• عَرَض = a symptom
• أعراض = symptoms

This is an example of a broken plural, which means the plural is formed by changing the internal pattern of the word rather than just adding a suffix.

That is very common in Arabic.

Arabic usually does not use a separate word for is/are in the present tense.

So:

• هذه الأعراض مهمة literally looks like these symptoms important
• but it naturally means these symptoms are important

This kind of sentence is called a nominal sentence.

If you wanted a past tense meaning, Arabic would use a verb, for example:

• كانت هذه الأعراض مهمة = these symptoms were important

That is a normal difference between Arabic and English.

In Arabic, after a demonstrative like هذا / هذه / هؤلاء, the noun is often still definite with الـ.

So Arabic says:

• هذه الأعراض

not because it means these the symptoms word-for-word in English, but because that is simply the normal Arabic structure.

Compare:

• هذا الكتاب = this book
• هذه السيارة = this car
• هذه الأعراض = these symptoms

So the الـ here is not a mistake; it is expected.

Because مهمة is the predicate of the sentence, not part of the noun phrase these symptoms.

So the structure is:

• هذه الأعراض = these symptoms
• مهمة = important

Together: These symptoms are important.

If you added الـ and said هذه الأعراض المهمة, that would usually mean:

• these important symptoms

That would be a noun phrase, not a complete sentence.

So this is a very useful contrast:

• هذه الأعراض مهمة = These symptoms are important
• هذه الأعراض المهمة = These important symptoms

For a learner, the easiest way to understand it is that هذه الأعراض works together as these symptoms.

So in practical terms:

• هذه الأعراض = subject
• مهمة = predicate

In more traditional grammar, Arabic grammarians may analyze the parts more precisely, but for learning and understanding the sentence, treating هذه الأعراض as one unit is the most helpful approach.

A careful full pronunciation would be approximately:

• hādhihi al-aʿrāḍu muhimmatun

In normal pause, many case endings are not pronounced, so you will often hear something closer to:

• hādhihi al-aʿrāḍ muhimma

A few pronunciation notes:

• هذه = hādhihi
• أعراض contains the letter ع, which has no exact English equivalent
• مهمة ends with ة, which is usually pronounced like -a when stopping

In fully vocalized MSA, the sentence is:

• هٰذِهِ الأَعْرَاضُ مُهِمَّةٌ

The endings are:

• الأعراضُ = nominative, because it is the subject
• مهمةٌ = nominative, because it is the predicate of a nominal sentence

In everyday reading and speech, these short final vowels are often not written or fully pronounced, but they are part of the underlying grammar.

In standard MSA, with a non-human plural like الأعراض, the normal and expected choice is the feminine singular adjective:

• الأعراض مهمة

Using a true plural adjective here would not be standard in formal Modern Standard Arabic.

So for learners, the safe rule is:

• non-human plural noun -> feminine singular agreement

That rule will help you with many other sentences too.