יש לנו הזמנה למסעדה בשעה 8, אז כדאי לצאת מוקדם.

Hebrew often expresses possession with יש + an indirect object.

So יש לנו הזמנה literally means there is a reservation for us / to us, which is how Hebrew says we have a reservation.

• יש לי = I have
• יש לך = you have
• יש לנו = we have

So you do not say something like אנחנו יש... for we have.

In this sentence, הזמנה means reservation.

But this word can also mean other things depending on context, especially:

• invitation
• order (for example, an order placed online or for food)

So context is very important.

Examples:

• הזמנה לחתונה = a wedding invitation
• הזמנה באינטרנט = an online order
• הזמנה למסעדה = a restaurant reservation / booking

Good question: in normal unpointed Hebrew writing, למסעדה can represent either:

• to a restaurant
• to the restaurant

That is because when ל combines with ה (the), the spelling usually looks the same without vowel marks.

So the exact meaning depends on context.

In this sentence, למסעדה is understood as part of the idea a reservation for the restaurant / at the restaurant. Native speakers may also say things like:

• יש לנו הזמנת מקום במסעדה
• יש לנו שולחן במסעדה

Those are more explicitly about a restaurant reservation.

Yes. Both are possible.

• בשעה 8 = at 8 o’clock
• בשמונה = at eight

בשעה 8 is a little more explicit and slightly more formal/neutral. בשמונה is very natural in everyday speech.

You may also see ב-8 in informal writing or texting.

Both can appear, but they are used a little differently.

• בשעה 8 = at 8 o’clock
• לשעה 8 = for 8 o’clock

With reservations, Hebrew often uses לשעה to emphasize the scheduled time of the booking:

• יש לנו הזמנה לשעה 8 = We have a reservation for 8 o’clock

In your sentence, בשעה 8 attaches naturally to the whole situation:

• We have a reservation..., at 8

So the given sentence is understandable, but לשעה 8 is also very common with reservations.

כדאי means something like:

• it’s advisable
• it’s a good idea
• it’s worth it

So כדאי לצאת מוקדם means it’s a good idea to leave early or more naturally in English, we should leave early.

It is softer than צריך (need to / must).

Compare:

• כדאי לצאת מוקדם = We should / it’s a good idea to leave early
• צריך לצאת מוקדם = We need to leave early / have to leave early

Because לצאת is the infinitive, meaning to leave / to go out.

After כדאי, Hebrew commonly uses an infinitive:

• כדאי ללכת = it’s a good idea to go
• כדאי לחכות = it’s a good idea to wait
• כדאי לצאת מוקדם = it’s a good idea to leave early

The ל־ at the beginning is the normal marker of the infinitive in Hebrew.

Because Hebrew often leaves the subject unstated when it is already clear from context.

The first part says יש לנו (we have / we’ve got), so it is obvious that we are the ones who should leave early.

If you want to make the subject more explicit, you could say:

• אז כדאי לנו לצאת מוקדם = so it’s a good idea for us to leave early
• אז כדאי שנצא מוקדם = so we should leave early

All of these are natural.

Hebrew often uses adjective forms as adverbs.

So מוקדם is literally the adjective early, but here it functions like the adverb early in English:

• לצאת מוקדם = to leave early

This is very common in Hebrew. In these adverb-like uses, Hebrew often uses the masculine singular form as the default form.

Here אז means so.

It connects the first clause to the conclusion:

• We have a reservation at 8, so it’s a good idea to leave early.

Depending on context, אז can also mean then, but in this sentence it is clearly the logical so.

It is understandable and fairly natural, but native speakers might also say it in a few other common ways.

For example:

• יש לנו הזמנת מקום במסעדה בשמונה, אז כדאי לצאת מוקדם.
• יש לנו שולחן במסעדה בשמונה, אז כדאי לצאת מוקדם.
• הזמנו מקום במסעדה לשמונה, אז כדאי לצאת מוקדם.

These versions are sometimes a little more explicit or a little more conversational, but your original sentence works well too.