حدة البصر مهمة.

With full case endings, it is:

حِدَّةُ البَصَرِ مُهِمَّةٌ

A careful pronunciation would be:

ḥiddatu l-baṣari muhimmatun

In normal pause pronunciation, you will often hear something closer to:

ḥiddat al-baṣar muhimmah

A useful thing to notice is that حِدَّة has ـة (tāʾ marbūṭa). In isolation it often sounds like -ah, but in a construction like this it is pronounced -at / -atu.

حِدَّة means sharpness, acuteness, or keenness.

By itself, it can describe the quality of being sharp or intense. In حِدَّةُ البَصَر, it means the sharpness of vision, so the whole phrase means visual acuity or sharp eyesight.

It comes from the root ح د د, which is associated with sharpness and edge.

Yes. حِدَّةُ البَصَر is an iḍāfa construction, often translated as the sharpness of sight or visual acuity.

In an iḍāfa:

• the first noun is the main thing being described
• the second noun tells you what it is related to

So here:

• حِدَّة = sharpness
• البَصَر = sight / vision

Together: sharpness of vision

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

So Arabic says:

• حِدَّةُ البَصَر = the sharpness of vision

not:

• الحِدَّةُ البَصَر ❌

The definiteness of the whole phrase is determined by the second noun. Since البَصَر is definite, حِدَّةُ البَصَر is definite as a whole.

Because it agrees with حِدَّة, and حِدَّة is a feminine noun.

• masculine: مُهِمّ = important
• feminine: مُهِمَّة = important

Since the subject is حِدَّةُ البَصَر and the head noun of that phrase is حِدَّة, the predicate must be feminine singular:

• حِدَّةُ البَصَرِ مُهِمَّةٌ

Arabic usually leaves out is / are in the present tense.

So:

• حِدَّةُ البَصَرِ مُهِمَّةٌ
• literally: Sharpness of vision important
• natural English: Sharp eyesight is important

This is called a nominal sentence.

If you wanted the past tense, you would add a verb such as كان:

• كانت حدة البصر مهمة = Visual acuity was important

With full grammatical endings:

حِدَّةُ البَصَرِ مُهِمَّةٌ

Here is what is happening:

• حِدَّةُ = nominative, because it is the first part of the subject
• البَصَرِ = genitive, because it is the second term of the iḍāfa
• مُهِمَّةٌ = nominative, because it is the predicate

So the structure is:

• subject: حِدَّةُ البَصَرِ
• predicate: مُهِمَّةٌ

In ordinary Arabic writing, these short vowel endings are usually not written.

It is definite as a whole.

Even though حِدَّة itself does not have الـ, the full iḍāfa becomes definite because the second noun is definite:

• البَصَر = the vision / sight
• therefore حِدَّةُ البَصَر = the sharpness of sight

But when translating into English, you often use a more natural phrase such as:

• Sharp eyesight is important
• Visual acuity is important

So Arabic definiteness does not always map directly to English the.

Arabic often uses an abstract noun where English might use an adjective.

So:

• حِدَّةُ البَصَرِ مُهِمَّةٌ = Sharp eyesight / visual acuity is important

This sounds formal and natural, especially in medical, educational, or general explanatory language.

You could also make a sentence with an adjective, such as:

• بصره حادّ = His eyesight is sharp

But that is a slightly different structure and often more personal or descriptive. The version with حِدَّة is more abstract and general.

البَصَر usually refers to sight / vision as a faculty or sense.

It is not exactly the same as:

• النظر = looking, gazing, or considering
• الرؤية = seeing, vision, or a view/viewpoint

In this sentence, البَصَر is the best fit because the phrase is about the quality of eyesight itself. That is why حِدَّةُ البَصَر is a natural expression for visual acuity.

Yes. Depending on context, natural English translations include:

• Sharp eyesight is important.
• Visual acuity is important.
• The sharpness of vision is important.
• Good eyesight is important.

The most literal translation is closer to The sharpness of sight is important, but in normal English, Sharp eyesight is important is usually the most natural.