Learn Hardcore Chinese: zuótiān wǒ gēn tā shuō wǒ bù néng qù.

Without: 昨天我跟她说我不能去。 (neutral past)
With: 昨天我跟她说了我不能去。 (emphasizes the telling was completed) Don’t use experiential 过 with a definite time like 昨天 (see below).

Why does the sentence have two 我? Can I drop the second one?

The second 我 is the subject of the reported clause (what you said). It’s normal to repeat it. If the subject is obvious, you can omit it:

昨天我跟她说我不能去。 (explicit)
昨天我跟她说不能去。 (subject of the embedded clause understood as “I”)

What does 跟 do here? Does it mean “with” or “to”?

Here 跟 is a coverb (preposition) marking the addressee: “say to someone.” So 我跟她说 means “I said/told her.” Literally it looks like “with her say,” but functionally it’s “to her.”

Can I use 对 or 给 instead of 跟 with 说?

对她说 is correct and a bit more formal or written.
跟她说 is very common in speech.
给她说 is regional/colloquial (heard in parts of the North). In standard use, prefer 跟/对; or switch the verb to 告诉: 告诉她…

What’s the difference between 说 and 告诉?

说 = say/speak. Structure: 跟/对 + 人 + 说 + 内容 or just 说 + 内容.
告诉 = tell/inform (requires a recipient). Structure: 告诉 + 人 + 内容. Note: 说她/说他 can mean “to criticize/scold her/him,” not “speak to her/him.”

Could/should I add quotation marks around what I said?

Optional. With quotes:

昨天我跟她说：“我不能去。” Without quotes (very common): 昨天我跟她说我不能去。

Why is 不 placed before 能? Could I use 没能?

Negation of modals goes before the modal:

不能 = cannot (ability/permission/possibility).
没能 = did not manage to (one-time outcome in the past). Examples:
昨天我跟她说我不能去。 (I can’t/am not allowed/it’s not possible to go.)
昨天我跟她说我没能去。 (I didn’t manage to go yesterday.)

What’s the difference among 能/可以/会/去不了/没法(办法)去?

能: can (ability/feasibility). 不能去 = cannot go.
可以: may (permission). 不可以去 = not allowed to go.
会: will/know how. 不会去 often reads as “won’t go,” not inability.
去不了: can’t make it (circumstantial impossibility; potential complement).
没法/没办法去: there’s no way to go (no means/solution). Pick based on nuance.

Why use 去 and not 来? Aren’t both “go/come”?

Chinese chooses 来 (come) or 去 (go) based on direction relative to the speaker (or the perspective adopted in the sentence).

If the event is away from the speaker: 去 (go).
If it’s toward the speaker’s location: 来 (come). In reported speech, you usually keep your current perspective: if the event was “there,” 我不能去 fits. If talking to someone at the destination, you might say 我不能来 (“I can’t come”).

Do I need to specify the destination after 去?

No. If the destination is understood from context, 去 alone is fine. Here it means “go (to that event/place).”

How do I pronounce 去 (qù)? Is that “u” actually ü?

Yes. After q/j/x, the written “u” is pronounced ü. So qù is “chyü” (with a tight lip rounding).

Does 不 change tone here (tone sandhi)? Should it be bú?

不 changes to second tone (bú) only before a fourth-tone syllable. Here it’s before 能 (néng, 2nd tone), so it stays bù: bù néng. Contrast: 不去 is bú qù (before a fourth tone).

Any tone-sandhi issues with 我 (wǒ)?

Yes: third tone before a non–third tone is pronounced as a “half” third (low, without the final rise). So 我跟 (wǒ gēn) is pronounced with a low wǒ. Pinyin spelling doesn’t change.

Can I omit the addressee and just say 我说…?

Yes, if the addressee is obvious or irrelevant: 昨天我说我不能去。 Caution: 我说她 usually means “I criticized her,” not “I talked to her.”

What’s the general word order with these elements (time, person told, verb, content)?

A common order is: time/topic > subject > coverb phrase (e.g., 跟她) > main verb > content. Example: 昨天 / 我 / 跟她 / 说 / 我不能去。

What does sentence-final 了 do in 我不能去了?

It marks a change of state/new situation: 昨天我跟她说我不能去了 = “I told her I can’t go anymore (my plan changed).” Neutral statement without that nuance: ……我不能去。

Can I use 过 here: 昨天我跟她说过我不能去?

Generally no. 过 marks an indefinite past experience and normally doesn’t pair with a specific time like 昨天. Use:

我跟她说过我不能去。 (at some time before)
Or with a count: 昨天我跟她说过一次我不能去。

Is 她 (she) pronounced the same as 他/它 (he/it)?

Yes, all are tā in speech. The difference appears only in writing; context disambiguates in conversation.

Is 和她说 okay?

Many speakers say 和她说话 for “talk with her,” but for “say/tell (someone) something,” standard Mandarin prefers 跟她说 or 对她说. For a clear “tell,” 告诉她 is safest.

Could I say 昨天我对她说我去不了? How does 去不了 differ from 不能去?

Yes. 去不了 (potential complement) emphasizes practical impossibility (can’t manage to make it), often due to circumstances (traffic, schedule). 不能去 is broader (not able/allowed/possible).