أفكر كثيرا في هذا القرار.

Because the verb أفكر already includes the subject I.

In Arabic, verbs are conjugated for person, number, and sometimes gender. The أـ at the beginning of أفكر marks first person singular in the present/non-past tense, so I is built into the verb.

A separate pronoun أنا can be added, but usually only for emphasis:

• أنا أفكر كثيرا في هذا القرار = I am thinking a lot about this decision.

Without emphasis, أفكر كثيرا في هذا القرار is perfectly normal.

Good question. In fully vocalized Arabic, this form is أُفَكِّرُ.

The reason you often see it written simply as أفكر is that normal Arabic writing usually leaves out:

• short vowels
• the shadda (the doubling mark), unless needed for clarity

So:

• fully marked: أُفَكِّرُ
• normal spelling: أفكر

This verb comes from Form II فكّر, which often has the sense of to think about / consider / reflect on.

It can mean either, depending on context.

Arabic present/non-past forms often cover both:

• simple present: I think
• present progressive: I am thinking

So أفكر can be understood as:

• I think
• I am thinking

The sentence context and the adverb كثيرا help shape the meaning. In many contexts, this sentence suggests repeated or ongoing thought.

Because this verb is commonly used with the preposition في when you mean think about something.

The pattern is:

• فكّر في شيء = to think about something

So in this sentence:

• أفكر في هذا القرار = I think/am thinking about this decision

This is one of those places where Arabic and English organize verbs differently. A verb-preposition combination often has to be learned as a unit.

Literally, في often means in, but after فكّر it is best understood as part of the expression فكّر في.

So although the individual word في usually means in, the whole phrase functions like:

• think about
• consider
• reflect on

This is very common in Arabic: a preposition may have a more idiomatic meaning depending on the verb it follows.

كثيرا is acting as an adverb, modifying the verb أفكر.

Here it means something like:

• a lot
• much
• sometimes, depending on context, often

So it tells you the degree/frequency of the thinking.

In fully vocalized Arabic, it would be written كثيرًا. That final ًا reflects an accusative adverbial form, but learners can first think of it simply as a very common way to say a lot after a verb.

In standard Arabic, when a noun follows a demonstrative like هذا (this), the noun is usually definite and takes الـ.

So the normal pattern is:

• هذا القرار = this decision
• هذه الفكرة = this idea

That is why القرار has الـ.

So:

• هذا قرار is generally not the standard pattern for this decision
• هذا القرار is the expected MSA structure

Because هذا is the demonstrative for a masculine singular noun, and القرار is masculine singular.

Arabic demonstratives agree with the noun in gender and number.

For example:

• هذا القرار = this decision
• هذه الفكرة = this idea
• هذان القراران = these two decisions
• هذه القرارات = these decisions

So هذا is the correct choice because قرار is masculine singular.

After a preposition like في, the noun is in the genitive case.

So the fully vocalized phrase would be:

• في هذا القرارِ

That final ـِ on القرار is the genitive ending.

In ordinary writing, case endings are usually not written, so you normally just see:

• في هذا القرار

But grammatically, it is still genitive.

This order is very natural and neutral, but Arabic word order is somewhat flexible.

The sentence as given:

• أفكر كثيرا في هذا القرار

sounds straightforward and natural.

Other orders are possible for emphasis, for example:

• أفكر في هذا القرار كثيرا
• في هذا القرار أفكر كثيرا

These can shift the focus slightly, but the original order is a very good standard choice for learners.

A careful MSA pronunciation would be:

ufakkiru kathīran fī hādhā al-qarār

A few pronunciation notes:

• أفكر = ufakkiru
  
  • the kk sound comes from the doubled consonant
• كثيرا = kathīran
  
  • ث sounds like th in think
• في = fī
  
  • long ee
• هذا = hādhā
  
  • ذ sounds like th in this
• القرار = al-qarār
  
  • ق in MSA is a deep q sound, not the English k
  • the final ā is long

If case endings are not being pronounced in pause, many speakers would say something closer to:

• ufakkir kathīran fī hādhā al-qarār