Learn Hardcore Chinese: zhōngwǔ wǒ yǒu shìqíng, yǐhòu wǒ zài dǎ diànhuà gěi nǐ. - I have things to do at noon; I’ll call you again later.

Because 有 is itself a verb meaning “to have.” In Chinese you say 我有事情 (I have something to do), not 我是有事情. The copula 是 isn’t used before verbs like 有 or adjectives.

What’s the difference between 有事情, 有事, and 很忙?

有事情/有事 both mean “I have something (to do)/I’ve got stuff going on,” and are a common, polite way to say you’re occupied.
很忙 says directly “I’m very busy.”
Softer: 有点事(情) “I’ve got a bit of something (to take care of).”

Why is 中午 placed at the start? Could I say 我中午有事情?

Time expressions often go before the subject or right after it. Both 中午我有事情 and 我中午有事情 are natural. Don’t add 在 here; 在中午 sounds awkward unless in specific formal contexts.

Why is 我 repeated in both clauses? Can I drop the second 我?

Repeating the subject is fine, but you can omit it in the second clause if it’s clear: 中午我有事情，以后再打电话给你。

What does 以后 mean here, and how is it different from 后来 or 之后?

Here 以后 means “later/afterwards (in the future).”

以后 = after now or after a reference time, often future-facing.
后来 = “later on” in past narratives only.
之后 = “after that,” a bit formal, can be past or future if the reference time is clear.

Why use 再 here? How is 再 different from 又?

再 is for actions to happen again/later in the future or as a next step: 以后再打.
又 is for repetition that’s already happened or is viewed as past/habitual: 他昨天又打电话了.

Where does 再 go in the sentence?

Put 再 right before the verb: (以后) 再 + 打(电话).
Correct: 以后我再打电话给你。
Incorrect: 我打再电话给你, 我再给你电话打.

Do I need 会 to mark the future, like 我以后会再打电话给你?

You don’t need it; Chinese often leaves future unmarked. Adding 会 emphasizes likelihood/intention: 我以后会再打电话给你 is also fine.

Which is more natural: 打电话给你 or 给你打电话? What about 打给你?

All are acceptable. Mainland speakers often say 给你打电话, while 打电话给你 and 打给你 are very common in Taiwan (and also understood in the mainland). Avoid 打给你电话 (redundant object) and 打你电话 (nonstandard); 打你的电话 means “call your number,” which is slightly different.

What is 给 doing here? Is it the verb “to give”?

Here 给 works like a preposition marking the recipient (a “coverb”): 给你打电话 = “call to you.” It introduces the indirect object, similar to English “to/for.”

Can I use 跟 instead of 给 (e.g., 跟你打电话)?

Use 给 for calling someone: 给你打电话/打电话给你. 跟 is used for doing something together or “with,” e.g., 跟你说话 (talk with you), 跟你通电话 (be on the phone with you).

Do I need a measure word with 电话? For example, 打个电话?

You can say 打个电话给你/给你打个电话 to sound a bit more casual or specific (“give you a call”). Without 个, 打电话 is also perfectly natural.

I confuse 再 and 在. Which one is used here and why?

It’s 再 (zài, fourth tone) meaning “again/later.” 在 (zài, fourth tone) means “at/in/on” or marks the progressive: 我在打电话 = “I am on the phone.” Here we need the “later” sense, so 再.

Is 中午以后 the same as 以后?

No. 中午以后 means “after noon (today),” a specific window. Bare 以后 is vaguer (“later/afterwards”). If you mean “soon,” say 等一下/一会儿/待会儿.

Could I use 然后 instead of 以后?

然后 means “and then/after that” in a sequence and often implies a relatively immediate next step. 以后 is looser in time and often means “later (not necessarily right away).” Both can work, but 以后 better fits a polite postponement.

Any pronunciation tips (tone sandhi) in this sentence?

Third-tone sandhi applies:

我有 wǒ yǒu → pronounced roughly as wó yǒu (2nd + 3rd).
给你 gěi nǐ → géi nǐ (2nd + 3rd).
Other words keep their listed tones: 中午 zhōngwǔ, 电话 diànhuà, 以后 yǐhòu, 再 zài, 打 dǎ.

Should there be a 了 at the end?

No. A sentence-final 了 marks a new situation or change; it’s not needed for a simple future intention like this. Just say 以后我再打电话给你 (optionally add 吧 to soften).

How can I make this even more polite or softer?

Add softeners or respectful forms:

中午我有点事，待会儿再给您打个电话吧。
不好意思，中午不太方便，我晚点再联系您。