بعض الأعراض مهمة.

Because this is a nominal sentence in the present tense. In Modern Standard Arabic, the verb to be is usually not expressed in the present.

So Arabic says:

• بعض الأعراض مهمة
• literally: some symptoms important

but in natural English that becomes some symptoms are important.

If you wanted the past or future, Arabic would normally use a verb such as كان:

• كانت بعض الأعراض مهمة = Some symptoms were important

بعض is a noun meaning some, part, or a portion.

In this sentence, it functions as the first part of an iḍāfa (possessive/genitive construction):

• بعضُ الأعراضِ

Literally, that is something like some of the symptoms or a part of the symptoms.

So بعض is not just a simple adjective like English some. Grammatically, it is a noun that is completed by the word after it.

Because بعض الأعراض is an iḍāfa construction.

In an iḍāfa:

• the first noun is the main word: بعض
• the second noun comes in the genitive: الأعراضِ

So, in fully vocalized form, you would write:

• بَعْضُ الأَعْراضِ

Here:

• بعضُ is nominative
• الأعراضِ is genitive because it is the second term of the iḍāfa

Usually, no. The normal and most direct structure is:

• بعض الأعراض

not

• بعض من الأعراض

Arabic often uses بعض + noun directly.

Using من can happen in some contexts, but in a simple sentence like this, بعض الأعراض is the standard phrasing.

This is a very common question.

After بعض, Arabic often uses a definite plural noun:

• بعض الأعراض

This can correspond in English to either:

• some symptoms
• or some of the symptoms

Which one sounds better depends on context.

So the presence of الـ does not always force you to translate it awkwardly. Arabic and English do not use definiteness in exactly the same way here.

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

So Arabic treats الأعراض for agreement purposes like a feminine singular noun. That is why the predicate is:

• مهمة

not a regular plural form.

This is a very important pattern:

• أعراض مهمة = important symptoms
• الكتب مفيدة = the books are useful
• البيوت كبيرة = the houses are big

All of those plural nouns are non-human, so the adjective/predicate appears in the feminine singular.

Not in this sentence. Here, مهمة is the predicate of the sentence, not just an adjective inside the noun phrase.

So:

• بعض الأعراض مهمة = Some symptoms are important

But compare that with:

• أعراض مهمة = important symptoms
• الأعراض المهمة = the important symptoms

So the same word مهمة can be:

• a normal adjective inside a noun phrase, or
• the predicate of a full sentence

The structure tells you which one it is.

In fully vocalized, formal grammar, it is:

• بَعْضُ الأَعْراضِ مُهِمَّةٌ

The endings are:

• بعضُ — nominative, because it is the subject/topic of the nominal sentence
• الأعراضِ — genitive, because it is the second part of the iḍāfa
• مهمةٌ — nominative, because it is the predicate

In normal reading aloud, especially when pausing at the end, those final short vowels are often not pronounced clearly.

A careful pronunciation is approximately:

• baʿḍu l-aʿrāḍi muhimmatun

If you pause naturally at the end, it often sounds more like:

• baʿḍ al-aʿrāḍ muhimmah

A few pronunciation notes:

• ع in الأعراض is the letter ʿayn, a throat sound with no exact English equivalent.
• ض in بعض and الأعراض is an emphatic consonant, heavier than ordinary d.
• ة in مهمة is pronounced -ah in pause, but -tun with full tanwīn in formal grammatical pronunciation.

It is a nominal sentence (جملة اسمية).

Its two main parts are:

• بعض الأعراض — the subject/topic side
• مهمة — the predicate/comment side

So the sentence is built as:

• [noun phrase] + [predicate]

This is one of the most basic and common sentence patterns in Arabic.