اللاعب يسجل هدفا.

A careful MSA pronunciation is al-lāʿibu yusajjilu hadafan.

A few notes:

• اللاعبُ = al-lāʿibu
• يُسَجِّلُ = yusajjilu
• هَدَفًا = hadafan
• The symbol ʿ represents the Arabic letter ع.
• The jj in yusajjilu shows a doubled consonant, because there is a shadda on ج.
• In everyday speech or when pausing, final short vowels are often not fully pronounced.

ال is the Arabic definite article, meaning the.

So:

• لاعب = player
• اللاعب = the player

Arabic does not use capitalization the way English does, so definiteness is shown with ال, not with a capital letter.

Because the noun لاعب begins with ل, and ل is one of the sun letters. When ال comes before a sun letter, the l of the article assimilates to the first consonant of the word.

So ال + لاعب is pronounced with a doubled l sound: al-lāʿib.

In fully marked text, this doubling is shown with a shadda on the second ل.

يسجل is in the imperfect form. In MSA, the imperfect can express:

• the present: scores / is scoring
• the future, if context or a future marker is added
• habitual action: usually scores

Here, without extra context, it can naturally mean scores or is scoring.

يسجل is the 3rd person masculine singular imperfect form of سجّل.

So it means:

• he scores
• he is scoring

The base verb سجّل often means to record or to register, but in sports contexts it commonly means to score.

Both are possible in MSA.

• اللاعب يسجل هدفًا starts with the subject: The player scores a goal.
• يسجل اللاعب هدفًا starts with the verb: The player scores a goal.

The difference is mainly one of style and emphasis:

• starting with اللاعب highlights or introduces the player
• starting with يسجل feels more verbally centered and is also very common

So the given sentence is perfectly normal; it just puts the subject first.

Because هدفًا is an indefinite direct object.

Here is the grammar:

• هدف = goal
• It is the object of يسجل, so it takes the accusative case
• Because it is indefinite, it takes tanween

That is why, in full spelling with case marking, you get هَدَفًا.

The final ا is just a spelling support for fatḥatayn; it does not mean you pronounce a long ā. You still say hadafan, not hadafā.

Arabic does not have a separate indefinite article like English a/an.

Instead, indefiniteness is usually shown by:

• the absence of ال
• and, in fully vocalized MSA, often by tanween

So:

• الهدف = the goal
• هدفًا = a goal (as an accusative indefinite noun)

In normal Arabic writing, short vowels and case endings are usually omitted.

If you fully vowel the sentence, it is:

اللّاعِبُ يُسَجِّلُ هَدَفًا

Those endings show the grammar:

• اللاعبُ ends in -u because it is the subject
• يسجلُ ends in -u because it is in the indicative
• هدفًا ends in -an because it is an indefinite accusative object

So the unvoweled sentence is normal spelling, and the fully voweled version shows the grammar more clearly.

Because the subject اللاعب is singular masculine: the player.

So the verb must also be 3rd person masculine singular:

• هو يسجل
• اللاعب يسجل

If the subject were plural, the verb form would change accordingly.