أوه، نسيت الموعد.

A careful MSA-style pronunciation is:

ʔōh, nasītu al-mawʿida
or, without full case endings in normal reading:
ʔōh, nasīt al-mawʿid

A few notes:

• أوه = oh
• نسيت is pronounced roughly na-seet in pause, or na-see-tu with the full ending
• الموعد is roughly al-maw-id, but with the Arabic sound ع in the middle: al-mawʿid

The hardest part for many English speakers is ع in موعد. If you cannot produce it well yet, that is normal.

In Arabic, the verb itself usually shows the subject.

نسيت means I forgot because the verb form already includes first person singular.

So Arabic does not need a separate pronoun here.
If you wanted to include I for emphasis, you could say:

أنا نسيت الموعد

But in normal Arabic, that is often unnecessary.

Both refer to the same verb form, but the second one shows the full grammatical ending.

• نسيت = the usual everyday spelling without short vowels
• نسيتُ = fully vocalized, showing the -u ending

In careful grammar, the full MSA form is:

نسيتُ الموعدَ

where:

• نسيتُ = I forgot
• الموعدَ = the direct object, with accusative ending -a

In most normal printed Arabic, these short vowel endings are omitted.

It is the past tense.

So نسيت literally means I forgot.

Depending on context, Arabic past tense can sometimes sound natural in English as:

• I forgot
• I have forgotten

But the basic form itself is simply the Arabic past tense.

The dictionary form is نَسِيَ.

That means to forget.

This verb is a little tricky because it is a defective verb, meaning its last root letter is weak. Its root is:

ن س ي

Because of that, some forms change in ways that may look unusual to learners.

For example:

• نَسِيَ = he forgot
• نَسِيتُ = I forgot
• يَنسَى = he forgets

So نسيت comes from the verb نَسِيَ.

الـ is the Arabic definite article, meaning the.

So:

• موعد = an appointment / a date / a scheduled time
• الموعد = the appointment / the date / the scheduled time

In your sentence, the speaker is talking about a specific appointment, so الـ is used.

Because م in موعد is a moon letter, not a sun letter.

In Arabic, the ل of الـ is:

• pronounced before moon letters
• assimilated before sun letters

Since م is a moon letter, you say:

الْمَوْعِد = al-mawʿid

not a doubled m sound.

موعد can mean several related things depending on context, such as:

• appointment
• date
• scheduled time
• meeting time

So in this sentence, it could mean:

• a doctor’s appointment
• a meeting
• a planned date/time
• sometimes even a romantic date, depending on context

In MSA, موعد is a broad and common word for an arranged time.

Arabic often prefers a sentence without an explicit subject pronoun when the verb already makes the subject clear.

So:

• نسيت الموعد = perfectly normal
• أنا نسيت الموعد = also correct, but more emphatic

Adding أنا can suggest emphasis such as:

• I forgot the appointment
  maybe contrasting with someone else

Without أنا, the sentence is more neutral and natural.

It would normally be accusative, because it is the direct object of the verb forgot.

So the fully inflected sentence is:

أوه، نسيتُ الموعدَ.

The ending -a on الموعدَ marks it as the object.

In most real-world writing, that ending is not written, so you usually just see:

أوه، نسيت الموعد.

Yes, أوه is an interjection, like English oh.

It expresses a reaction such as:

• surprise
• sudden realization
• regret
• mild frustration

So in this sentence, it signals that the speaker has just realized something:

Oh, I forgot the appointment.

Interjections like this are common in Arabic too, though the exact spelling can vary.

The sentence is fully understandable and acceptable in Modern Standard Arabic.

In everyday spoken Arabic, many dialects would use different wording. For example, a speaker might use a dialect verb instead of نسيت or a different word for appointment.

But as MSA, this sentence is natural and clear.

A fully formal version with case endings would be:

أوه، نسيتُ الموعدَ.

A more typical unvocalized written version is:

أوه، نسيت الموعد.

Yes, ع is part of the word and should ideally be pronounced.

The word is موعد = mawʿid.

For English speakers, ع is often one of the hardest Arabic sounds. Even if your pronunciation is imperfect at first, it is still worth trying, because:

• it is a real consonant in Arabic
• leaving it out completely can make your pronunciation less accurate
• it helps distinguish words

So yes, ع matters here.

Yes, in natural English that may be the best translation in some contexts.

Arabic نسيت الموعد literally says I forgot the appointment, but English often prefers:

• I forgot the appointment
• I forgot about the appointment

The Arabic structure does not require a separate word for about here. The verb simply takes the object directly.

Yes.

لقد نسيت الموعد is also correct and more explicitly marks the action as completed or realized.

Compare:

• نسيت الموعد = I forgot the appointment
• لقد نسيت الموعد = I really did forget the appointment / I have forgotten the appointment

In many contexts, لقد adds a slightly more formal or emphatic tone. The simpler sentence without it is very common.