إذا نسيت البطاقة مرة أخرى، فلن أستطيع أن أستعمل هذا الحساب.

This is a very common feature of Arabic conditionals.

After إذا in Modern Standard Arabic, the verb is often in the past tense form, even when the meaning is future in English.

So:

• إذا نسيت = literally something like if I forgot
• but naturally it means if I forget

This is normal Arabic usage. The past form here does not mean the action already happened; it is just the standard structure in many conditional sentences.

In normal Arabic writing, short vowels are usually not written, so نسيت can be ambiguous by itself.

It could represent:

• نسيتُ = I forgot
• نسيتَ = you forgot (masculine singular)
• نسيتِ = you forgot (feminine singular)

In this sentence, the later part is فلن أستطيع = then I will not be able, which is clearly first person singular. That makes it clear that نسيت here is understood as نسيتُ: if I forget.

Both can mean if, but they are not identical.

A useful beginner-level distinction is:

• إذا often introduces a condition that is seen as realistic, expected, or possible in a normal way
• إن is often more neutral, general, or sometimes a bit more uncertain

So in this sentence, إذا works well because it is talking about a real practical situation:

• إذا نسيت البطاقة مرة أخرى...
  If I forget the card again...

This sounds natural for a likely real-life event.

The فـ here connects the result of the condition to the if-clause.

So:

• إذا نسيت البطاقة مرة أخرى = if I forget the card again
• فلن أستطيع... = then I will not be able...

That فـ is very common in the answer/result part of a conditional sentence. In traditional grammar, this is part of the جواب الشرط structure.

It is especially expected when the result clause begins with words like:

• لن
• لا
• قد
• a command
• a nominal sentence

So فلن أستطيع is more grammatically complete than just لن أستطيع in this kind of sentence.

لن is used to negate a verb in the future.

So:

• أستطيع = I can / I am able
• لن أستطيع = I will not be able

It is stronger and more specifically future-oriented than simple لا.

Also, لن causes the following imperfect verb to be in the subjunctive. In fully vocalized Arabic, that affects the ending, but in normal writing you usually do not see that clearly.

After verbs like أستطيع (I can / I am able), Arabic very often uses أن + imperfect verb.

So:

• أستطيع أن أستعمل = I can / am able to use

This is a very standard pattern in MSA.

Here:

• أستطيع = I am able
• أن أستعمل = to use

Like لن, أن also puts the following imperfect verb into the subjunctive.

So the structure is:

• لن أستطيع أن أستعمل هذا الحساب
• I will not be able to use this account

Yes, it is a very normal way to say again.

Literally:

• مرة = time / occasion
• أخرى = other / another

So مرة أخرى literally means another time, and idiomatically it means again.

In this sentence:

• إذا نسيت البطاقة مرة أخرى
• If I forget the card again

This is a very common expression in both MSA and everyday Arabic.

Because الحساب is a masculine singular noun.

In Arabic, demonstratives must agree with the noun:

• هذا = this (masculine singular)
• هذه = this (feminine singular)

So:

• هذا الحساب = this account
• هذه البطاقة = this card

Notice that in the same sentence, البطاقة is feminine, so if you used this with it, you would say هذه البطاقة.

البطاقة means the card, not just a card.

Using الـ suggests a specific card that both speaker and listener can identify from context, for example:

• a bank card
• an ID card
• an access card

So:

• نسيت البطاقة = I forgot the card
• نسيت بطاقة = I forgot a card

In this sentence, the definite form sounds more natural because it is probably referring to a particular known card needed to access the account.

الحساب is a broad word that can mean:

• account
• bank account
• user account
• online account
• calculation (in other contexts)

In this sentence, the surrounding context suggests a practical account that requires a card, so the intended meaning is probably something like:

• this account
• possibly this bank account or this user/account-access system

Arabic often leaves this kind of detail to context.

No. أستعمل is correct and standard, but other verbs are also possible depending on style.

For example:

• أستعمل = I use
• أستخدم = I use

Both are common. In many modern contexts, learners will hear أستخدم very often, but أستعمل is perfectly good MSA.

So this sentence could also be phrased with أستخدم, but the original sentence is completely natural.

Yes, it is very normal.

The structure is:

• إذا نسيت البطاقة مرة أخرى
  If I forget the card again
• فلن أستطيع أن أستعمل هذا الحساب
  then I will not be able to use this account

This is a standard conditional pattern:

1. إذا
   • verb clause
2. result clause, often introduced with فـ

So the sentence is not unusual or poetic; it is straightforward standard Arabic.