هو متأكد.

In Arabic, the verb to be is usually not stated in the present tense.

So:

• هو متأكد = He is sure / He is certain

Arabic often just puts the subject and the predicate side by side in the present:

• هو طالب = He is a student
• هي مشغولة = She is busy

If you want the past or future, Arabic does use a verb:

• كان متأكدًا = He was sure
• سيكون متأكدًا = He will be sure

هو means he.

In هو متأكد, it is the subject of the sentence: he.

In many real contexts, Arabic can omit subject pronouns if the meaning is already clear, but with this kind of nominal sentence, هو is very natural and common when you want to explicitly say he.

So:

• هو متأكد = He is sure
• متأكد by itself can sometimes be understood from context, but it is less complete on its own as a full neutral sentence.

متأكد is an active participle used like an adjective, meaning sure or certain.

In this sentence, it describes هو, so it works as the predicate:

• هو = the subject
• متأكد = what is being said about him

You can think of it as similar to an adjective in English:

• هو متعب = He is tired
• هو جاهز = He is ready
• هو متأكد = He is sure

Because it matches هو, which refers to one male person.

In Arabic, words like متأكد must agree with the person they describe in gender and often number.

So you get:

• هو متأكد = He is sure
• هي متأكدة = She is sure
• هم متأكدون = They are sure (masculine/mixed)
• هن متأكدات = They are sure (feminine)

Agreement is very important in Arabic.

It is pronounced approximately:

mu-ta-ʾak-kid

A more careful fully vowelled form is:

مُتَأَكِّد

A few pronunciation notes:

• مُ = mu
• تَ = ta
• أ contains a hamza (a glottal stop), so there is a little break before that vowel
• كِّ has a shadda, so the k sound is doubled
• د ends the word

So it is not just a smooth mutaakid. There is a slight stop in the middle because of the hamza: mu-ta-ʾak-kid.

Because here it is an indefinite predicate, not the sure one.

In هو متأكد, the meaning is simply:

• He is sure
• not He is the sure one

Compare:

• هو متأكد = He is sure
• هو المتأكد = He is the one who is sure / He is the certain one

The version without الـ is the normal one for this meaning.

Yes. In careful full MSA, the sentence can be written as:

هُوَ مُتَأَكِّدٌ

The final ـٌ on متأكد is the nominative ending for an indefinite predicate.

However, in normal unvowelled writing, those endings are usually not shown:

• هو متأكد

And in pause, speakers often do not pronounce the final case vowel clearly anyway.

So learners should know both facts:

• grammatically: مُتَأَكِّدٌ
• in ordinary writing: متأكد

The usual MSA way is:

هو ليس متأكدًا

A few useful points:

• ليس means is not
• After ليس, the predicate is usually in the accusative, so in full vocalization you get متأكدًا

So:

• هو متأكد = He is sure
• هو ليس متأكدًا = He is not sure

You may also hear Arabic continue with من:

• هو ليس متأكدًا من الجواب = He is not sure of the answer

A very common pattern is:

هو متأكد من... = He is sure of/about...

Examples:

• هو متأكد من الأمر = He is sure about the matter
• هو متأكد من النتيجة = He is sure of the result
• هو متأكد من أنه على حق = He is sure that he is right

So if you want to expand the sentence, من is often the next word to use.