نمو الجذر سريع.

It is a nominal sentence in Modern Standard Arabic.

• نمو الجذر = the subject/topic
• سريع = the predicate/comment

More specifically, نمو الجذر is an iḍāfa phrase, meaning growth of the root or root growth.

So the sentence structure is basically:

• [the growth of the root] [is fast]

In present-tense nominal sentences, Arabic usually does not use an explicit verb for to be.

So:

• نمو الجذر سريع
• literally looks like root growth fast
• but means The growth of the root is fast

If you want past or future, then Arabic normally adds a verb such as كان or سيكون.

Because نمو الجذر is an iḍāfa construction.

In an iḍāfa:

• the first noun does not take الـ
• the first noun does not take tanwīn
• the second noun is put in the genitive

So نمو الجذر is correct.

Also, even though نمو itself has no الـ, the whole phrase can still be definite because the second noun is definite:

• الجذر = the root
• therefore نمو الجذر = the root’s growth / the growth of the root

Because it is the second term of the iḍāfa.

That second noun regularly carries a genitive ending in full MSA pronunciation. Its role is like of the root in English.

So:

• نمو الجذر = growth of the root

This is one of the most important patterns in Arabic noun phrases.

Because سريع here is the predicate, not an attributive adjective inside the noun phrase.

The sentence means:

• The growth of the root is fast

It does not mean:

• the fast root

If you wanted the fast root, you would say:

• الجذر السريع

Predicates in nominal sentences are often indefinite, so سريع without الـ is exactly what you would expect here.

Because the sentence is divided like this:

• نمو الجذر = one complete noun phrase
• سريع = what is being said about that noun phrase

So سريع describes the growth, not the root.

A useful contrast:

• نمو الجذر سريع = The root’s growth is fast
• الجذر سريع = The root is fast
• الجذر السريع = the fast root

Because it agrees with نمو, which is the head noun of the subject phrase.

• نمو is a singular masculine noun
• so the predicate is also singular masculine: سريع

Even though English may focus on root, Arabic agreement here follows the grammatical head of the phrase, which is نمو.

No. Here نمو is a noun, specifically a maṣdar or verbal noun.

It is related to the verb:

• نما = to grow
• ينمو = he/it grows

So Arabic is using a noun meaning growth rather than a finite verb meaning grows.

In fully inflected MSA, you could write it as:

• نموُّ الجذرِ سريعٌ

Functionally:

• نموُّ = nominative, because it is the subject/topic
• الجذرِ = genitive, because it is the second term of the iḍāfa
• سريعٌ = nominative, because it is the predicate

In ordinary Arabic writing, these short-vowel endings are usually omitted.

The dictionary form is commonly vocalized نُمُوّ and pronounced roughly numuww.

In this sentence, with nominative inflection, it is نُمُوُّ.

This can be confusing because normal Arabic spelling often leaves out the short vowels and shadda, so learners just see نمو.

Yes. That is a very natural way to think of it.

Arabic iḍāfa often corresponds to English:

• X of Y
• or a compact noun phrase like Y X

So نمو الجذر can be understood as either:

• the growth of the root
• root growth

Yes. A verbal version would be something like:

• ينمو الجذر بسرعة

That means The root grows quickly.

The sentence you were given, نمو الجذر سريع, is more noun-based:

• The growth of the root is fast

So the two are related, but the grammar is different.

For the past:

• كان نمو الجذر سريعًا = The growth of the root was fast

For the future:

• سيكون نمو الجذر سريعًا = The growth of the root will be fast

Notice the change in the predicate:

• in the original nominal sentence: سريعٌ
• after كان or سيكون: سريعًا

That happens because the predicate of كان becomes accusative.