نصف ساعة يكفي.

It is an iḍāfa construction: نصف + ساعة.

• نصف = half
• ساعة = hour

Literally, it is half of an hour, but in natural English it corresponds to half an hour. In this structure, نصف is the main noun, and ساعة completes its meaning.

Because after نصف, Arabic normally uses a singular noun in this kind of expression.

So Arabic says:

• نصف ساعة = half an hour
• نصف يوم = half a day
• نصف كيلو = half a kilo

The idea is half of one unit, so the noun stays singular.

Because the phrase is indefinite: it means half an hour, not half of the hour.

Compare:

• نصف ساعة = half an hour
• نصف الساعة = half of the hour / half the hour

Also, in an iḍāfa, the first noun usually does not take الـ. The definiteness of the whole phrase depends mainly on the second noun.

Because نصف is the first part of an iḍāfa (مضاف), and the first noun in an iḍāfa does not take tanwīn.

So in full vocalization, it is:

نِصْفُ سَاعَةٍ

not نِصْفٌ سَاعَةٍ.

The second noun, ساعةٍ, is in the genitive because it is the second part of the iḍāfa.

Because the phrase is being treated as a singular amount, and the grammatical head of نصف ساعة is نصف, which is masculine singular.

So يكفي is natural here.

You may also see تكفي in real usage, especially when the speaker feels the agreement more strongly with ساعة because it is feminine. But in نصف ساعة يكفي, the masculine يكفي is completely normal and standard.

Fully vocalized, it is:

نِصْفُ سَاعَةٍ يَكْفِي

Here is why:

• نِصْفُ is nominative, because it is the topic/subject-like part of the sentence
• سَاعَةٍ is genitive, because it is the second noun in the iḍāfa
• يَكْفِي is the imperfect verb to be enough / to suffice

In normal Arabic writing, these short vowels are usually omitted.

Yes. That is also a normal sentence.

Both are possible:

• نصف ساعة يكفي
• يكفي نصف ساعة

The difference is mostly one of focus or style:

• نصف ساعة يكفي highlights the amount first
• يكفي نصف ساعة feels more like a straightforward verbal sentence

Both can mean the same thing.

Because يكفي can stand on its own when the context already makes that clear.

If you want to state it explicitly, Arabic can add a pronoun or phrase, for example:

• يكفيني نصف ساعة = half an hour is enough for me
• نصف ساعة تكفيني = half an hour is enough for me
• نصف ساعة تكفي لهذا = half an hour is enough for this

So the original sentence is simply leaving that information understood from context.

The dictionary form is كَفَى.

Its basic pattern is:

• past: كَفَى
• imperfect: يَكْفِي

It means to suffice, to be enough, or sometimes to satisfy a need.

So يكفي here means is enough or suffices.

Learners may see it explained either way, but a useful beginner-friendly way to understand it is:

• نصف ساعة = the topic
• يكفي = the comment about it

So you can think of it as a topic-comment sentence: As for half an hour, it is enough.

You do not need to worry too much about the label at first. The important point is that Arabic often puts the amount first and then comments on it with a verb like يكفي.

With full case endings, it is pronounced approximately:

niṣfu sāʿatin yakfī

In less formal pronunciation, where case endings are not pronounced, you will often hear something closer to:

niṣf sāʿa yakfī

A pronunciation detail many learners notice is the letter ع in ساعة. That sound does not exist in English, so it may take practice.