أفهم بالكامل.

In fully vowelled MSA, it is أَفْهَمُ بِالْكَامِلِ.

A careful pronunciation is roughly:

• af-ha-mu
• bil-kā-mi-li

If you stop at the end of the sentence, the final short vowel is usually dropped in pronunciation, so you will often hear:

• afhamu bil-kāmil

A rough English-style guide would be AF-ha-mu bil-KAA-mil.

أفهم means I understand.

It comes from the root ف-ه-م, which is related to understanding and comprehension. The basic past-tense verb is فَهِمَ = he understood. From that, the present/imperfect أفهم means I understand.

The أ at the beginning marks first person singular in the present/imperfect tense.

So:

• أفهم = I understand
• تفهم = you understand or she understands depending on context
• يفهم = he understands
• نفهم = we understand

So the أ here is not a separate word. It is part of the verb and tells you the subject is I.

Because Arabic verbs already include the subject information.

So أفهم already means I understand, and you do not need أنا unless you want emphasis or contrast.

For example:

• أفهم بالكامل. = I understand completely.
• أنا أفهم بالكامل. = I understand completely / I do understand completely with extra emphasis on I

Both are correct, but the version without أنا is very normal.

It is the imperfect form, which in Arabic often covers the present and sometimes the future, depending on context.

In this sentence, it means I understand in the present.

If you wanted to make the future clearer, you could say:

• سأفهم بالكامل. = I will understand completely
• سوف أفهم بالكامل. = I will understand completely

But in your sentence, the natural meaning is present: I understand completely.

بالكامل means completely, entirely, or in full.

It is made from:

• بـ = a preposition, often with / by / in
• الكامل = the complete / the whole

Together, بالكامل is an idiomatic expression meaning completely or fully.

So the sentence is literally something like:

• I understand in full

but the natural English meaning is simply:

• I understand completely

Because the preposition بـ attaches directly to the following word in Arabic spelling.

So:

• بـ + الكامل becomes بالكامل

This is very common in Arabic. Other examples are:

• بالبيت = in the house
• باللغة العربية = in Arabic

So although it contains parts, it is normally written together.

Because بـ is a preposition, and prepositions in Arabic make the following noun genitive.

So in full vowelled form:

• بِالْكَامِلِ

The last vowel is -i because of the genitive case.

However, in normal pause at the end of a sentence, that final short vowel is usually not pronounced. So you will often hear:

• بِالْكَامِلْ

This is normal in spoken reading of MSA.

Yes. There are several common options, depending on style.

For example:

• أفهم تمامًا. = I understand completely.
• أفهم كليًا. = I understand completely / entirely.
• أفهم جيدًا. = I understand well.
  This is slightly different in meaning from completely.

بالكامل is correct and understandable, but تمامًا is often especially common for completely.

Both are okay.

By itself, أفهم بالكامل. means the thing understood is already clear from context, or it is left general.

If you want to name what is understood, you can add an object:

• أفهم الدرس بالكامل. = I understand the lesson completely.
• أفهم الفكرة بالكامل. = I understand the idea completely.
• أفهمه بالكامل. = I understand it completely.

So the shorter sentence is perfectly natural when the context already tells you what is being understood.

This sentence is perfectly good Modern Standard Arabic.

In everyday spoken Arabic, people often use dialect forms instead, and those forms vary by region. So for a learner of MSA, أفهم بالكامل. is correct and useful, especially in writing, news, formal speech, and classroom Arabic.

In conversation, native speakers may prefer different dialect wording, but for MSA this sentence is natural and correct.