هذا مسمار.

Because Arabic often makes simple present-tense sentences without a verb like is / am / are.

So:

• هذا = this
• مسمار = a nail

Together, هذا مسمار literally looks like this nail, but in standard grammar here it means:

• This is a nail.

This kind of sentence is called a nominal sentence in Arabic.

If you wanted to say This was a nail, then Arabic would use a verb:

• كان هذا مسمارًا = This was a nail.

هذا means this for a masculine singular noun.

Since مسمار is a masculine singular noun, هذا is the correct demonstrative.

Compare:

• هذا مسمار = This is a nail.
• هذه سيارة = This is a car. (because سيارة is feminine)

So one important thing to learn is that Arabic this/that words change according to gender and number.

In Arabic, every noun has grammatical gender, usually masculine or feminine.

مسمار is simply a masculine noun by dictionary gender.

A lot of feminine nouns have the ending ة (taaʾ marbuuṭa), but مسمار does not, and it is treated as masculine.

So Arabic uses:

• هذا مسمار not
• هذه مسمار

Because مسمار here is indefinite:

• مسمار = a nail
• المسمار = the nail

That gives two different meanings:

• هذا مسمار = This is a nail.
• هذا المسمار = This is the nail / This nail

So in your sentence, the noun is indefinite because it is identifying what the thing is, not pointing to a specific already-known nail.

This is a very common question.

1. هذا مسمار

This is a full sentence:

• This is a nail.

Here:

• هذا is the subject-like part
• مسمار is the predicate-like part

The noun is indefinite, so it tells you what something is.

2. هذا المسمار

This is usually understood as:

• this nail or
• this is the nail

Here the noun is definite because of ال.

So the basic contrast is:

• indefinite noun after هذا → often This is a ...
• definite noun after هذا → often this/the ...

A careful pronunciation is:

• hādhā mismārun

In normal pause, you will often hear:

• hādhā mismār

A few pronunciation notes:

• ها in هذا has a long ā sound: hā
• ذ is like the th in this
• the final ـار in مسمار has a long ā sound: -mār

So roughly for an English speaker:

• hā-dhā mis-mār

Not perfect English spelling, but close enough to start.

Yes — in fully vocalized formal Arabic, it would usually be written:

• هذا مِسْمَارٌ

That final ـٌ is called tanwīn ḍamm and often corresponds to an -un ending.

So in full grammar:

• hādhā mismārun

However, in normal reading and speech, especially when pausing at the end of the sentence, the final -un is usually not pronounced:

• hādhā mismār

So both of these ideas are important:

• Grammar form: مسمارٌ
• Pausing pronunciation: مسمار

Because in a basic nominal sentence like this, the predicate is typically in the nominative.

In traditional grammar:

• هذا = the topic / subject-like part
• مسمارٌ = the predicate (خبر)

And the predicate takes the nominative case.

That is why the full form is:

• هذا مسمارٌ

not

• هذا مسمارٍ
• هذا مسمارَ

Those other endings would belong to different grammatical contexts.

In this exact form, it is normally understood as a sentence:

• This is a nail.

Why? Because the noun مسمار is indefinite.

In Standard Arabic, when هذا is followed by an indefinite noun, it commonly forms a complete identifying sentence.

If you want the phrase this nail, Arabic normally uses a definite noun:

• هذا المسمار = this nail

So:

• هذا مسمار → sentence
• هذا المسمار → noun phrase (or a definite identification)

Not for the same meaning in normal Modern Standard Arabic.

If you want to say:

• This is a nail

the natural form is:

• هذا مسمار

If you want to say:

• this nail

the normal MSA form is:

• هذا المسمار

Putting هذا after the noun like مسمار هذا is not the standard way to say either of those in regular MSA.

So for learners, the safest pattern is:

• هذا + indefinite noun → This is a ...
• هذا + definite noun → this ...

Then you would usually use هذه instead of هذا.

For example:

• هذه سيارة = This is a car.
• هذه شجرة = This is a tree.

So the demonstrative must match the noun’s gender:

• masculine singular → هذا
• feminine singular → هذه

Since مسمار is masculine, the correct sentence is:

• هذا مسمار

In MSA, you would use ذلك for that (masculine singular):

• ذلك مسمار = That is a nail.

And for feminine singular:

• تلك سيارة = That is a car.

So the pattern is very similar:

• هذا مسمار = This is a nail.
• ذلك مسمار = That is a nail.

A simple way is:

• هذا ليس مسمارًا = This is not a nail.

Notice two things:

1. Arabic now uses ليس for negation.
2. مسمار becomes مسمارًا in this structure.

So:

• هذا مسمار = This is a nail.
• هذا ليس مسمارًا = This is not a nail.

This is a useful pattern to memorize early.