هذا الموعد مثالي.

هذا means this for a masculine singular noun.

It is used here because الموعد is a masculine singular noun.
If the noun were feminine, Arabic would normally use هذه instead.

So:

• هذا الموعد = this appointment / this date
• هذه... would be for a feminine noun

In Modern Standard Arabic, a demonstrative like هذا is commonly followed by a definite noun, so the noun usually keeps الـ.

So هذا الموعد is the normal way to say this appointment or this date.

To an English speaker, it may look like this the appointment, but that is just how Arabic structures it. It is completely standard.

الموعد can mean:

• the appointment
• the موعد / scheduled time
• sometimes the date in the sense of a planned meeting or arranged time

It usually refers to a scheduled time or appointment, not necessarily a romantic date.

The base noun is موعد, and الـ makes it definite:

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

Because Arabic usually does not use a present-tense form of the verb "to be" in sentences like this.

So:

• هذا الموعد مثالي literally looks like this appointment ideal
• but it means this appointment is ideal

This kind of sentence is very common in Arabic.

If you wanted the past or future, Arabic would normally use a verb such as كان:

• كان هذا الموعد مثاليًا = This appointment was ideal

This is a nominal sentence in Arabic, called جملة اسمية.

It has two main parts:

• هذا الموعد = the topic / subject part
• مثالي = the predicate, telling you something about it

Traditional grammar often analyzes this as:

• مبتدأ = the starting element
• خبر = the information given about it

So the structure is essentially:

• this appointment
  • ideal

Because here مثالي is the predicate of the sentence, not an adjective directly attached to the noun.

Compare these:

• هذا الموعد مثالي = This appointment is ideal
• الموعد المثالي = the ideal appointment

That is an important difference.

In الموعد المثالي, المثالي is an adjective describing الموعد, so it matches it in definiteness and takes الـ.

In هذا الموعد مثالي, مثالي is not part of the noun phrase the ideal appointment. It is the predicate: is ideal.

Yes. As an adjective/predicate adjective, مثالي agrees with the noun in gender and number.

Since الموعد is masculine singular, the form is مثالي.

For example:

• masculine singular: مثالي
• feminine singular: مثالية

So if the noun were feminine, you would expect مثالية instead.

A simple pronunciation is:

hādhā al-mawʿid mithālī

In very formal Arabic with full case endings, it would be:

hādhā al-mawʿidu mithāliyyun

A few useful notes:

• هذا = hādhā
• الموعد begins with m, so the l in الـ is pronounced clearly: al-mawʿid
• مثالي has a long ī sound at the end

In normal reading and in pause, final case endings are often not pronounced.

Yes, but the meaning changes slightly.

• هذا الموعد مثالي = This appointment is ideal
• هذا موعد مثالي = This is an ideal appointment

The difference is the presence or absence of الـ on موعد:

• الموعد = the appointment
• موعد = an appointment

So the original sentence talks about a specific, definite appointment and says that it is ideal.

A fully vowelled version is:

هٰذا الموعدُ مثاليٌّ

Here is what the endings show in careful formal Arabic:

• الموعدُ has ـُ because it is in the nominative position
• مثاليٌّ also has a nominative ending because it is the predicate of a nominal sentence

In most everyday printed Arabic, these short vowels are omitted, so you usually just see:

هذا الموعد مثالي