Learn Hardcore Chinese: Wǒ gāngcái wàngjì dài shū le, tā bāng wǒ kāi mén. - I just forgot to bring the book; he helped me open the door.

刚才 (gāngcái) means “just now / a moment ago” and refers to a recent past time (not the future).
Placement: It can go at the very start or after the subject:
- 刚才我忘记带书了。
- 我刚才忘记带书了。
  Both are natural.

Why is there a 了 at the end of 忘记带书了? What is it doing?

That 了 is sentence-final 了 (often called “了2”), marking a completed event or a change of situation. 我刚才忘记带书了 means “At that point (just now), the forgetting happened (and that’s now a fact).” It’s not the same as the verb-suffix 了 that attaches directly to a verb.

Can I say 忘了带书 instead? Which is more natural: 忘记 or 忘?

Yes: 我刚才忘了带书 is very natural.

忘 (wàng) is more colloquial; 忘记 (wàngjì) is slightly more formal or careful.
Patterns you’ll hear a lot:
- 忘了 + [object/VP] → 我忘了带书。
- 忘记 + [object/VP] → 我忘记带书了。
  Avoid 忘记了带书; it’s less natural.

Do I need a measure word with 书 here?

Not necessarily. 带书 can mean “bring (my) book(s)” in general. Use a measure word when you want to be specific:

带那本书 (bring that book)
带一本书 (bring one book)

What’s the difference between 带 and 拿 (and 带来/带去)?

带 (dài): bring/take along with you (focus on accompanying you).
拿 (ná): hold/carry in the hand (focus on the physical act).
带来/带去 add direction: bring here / take there.
Here, 忘记带书 = “forgot to bring the book (along).”

Does 他帮我开门 mean he opened the door for me, or that he assisted me while I opened it?

Usually it means “He opened the door for me” (he did it as help to me). Context can allow the “assisted me while I opened it” reading, but the beneficiary reading is the default.

Can I use 给 or 帮忙 instead of 帮 here?

他给我开门 is fine and means “He opened the door for me,” without emphasizing “help,” just doing it for my benefit.
他帮忙开门 is also fine. Note: 帮忙 is intransitive; you can’t say “帮忙我,” but you can say 帮我.
So:
✅ 他帮我开门 / 他给我开门 / 他帮忙开门
❌ 他帮忙我开门

Why doesn’t the second clause have 了? Should it be 他帮我开了门 or 他帮我开门了?

All are possible; the nuance differs slightly.

他帮我开门: plain statement; past is clear from context.
他帮我开了门: emphasizes the completion of the opening action.
他帮我开门了: sentence-final 了 marks the new situation/result.
Given the first clause sets a past-time context, leaving out 了 in the second clause is fine.

Would it be better to connect the two clauses with 所以, 于是, or 然后?

You can, if you want to show a clearer relation:

我刚才忘记带书了，所以他帮我开门。 (so)
……，然后他帮我开门。 (then)
Note: Forgetting a book doesn’t obviously lead to opening a door, so a more typical causal pair would be “忘带钥匙…开门.” Without a connector, Chinese often just places clauses side by side.

Is the sentence more natural with 钥匙 instead of 书?

For a cause–effect relation, yes:

我刚才忘记带钥匙了，他帮我开门。
For “书,” the two events can still be separate in time: “I forgot my book (earlier); later he opened the door for me.”

Could I use a 把 sentence here?

Yes:

我刚才把书忘了。 (I forgot the book.)
Add location: 我刚才把书忘在家里了。
For the door: 他把门开了 / 他把门打开了。 Using 把 puts focus on the affected object and the result.

What’s the difference between 开门 and 打开门?

开门 is a set verb-object for “open the door”; it’s very common and neutral. It can also mean “open for business” (商店开门了).
打开门 emphasizes the action of “opening it up” (more result-oriented; slightly more formal/literal).

Is 刚才 the same as 刚 or 刚刚?

They’re close but not identical:

刚才 is a time word (a moment ago).
刚 / 刚刚 are adverbs meaning “just.”
Often interchangeable in casual speech, but grammatically 刚才 behaves like a time expression, while 刚/刚刚 modify the verb.

Can I omit 我 in 他帮我开门?

No, not if you want to mark the beneficiary. 他帮开门 is incorrect. If you don’t mention the beneficiary, use 他开门了 or 他帮忙开门.

Why not use 过 here, like 忘记过带书?

过 marks past experience (“have ever”). 忘记过带书 would mean “I have had the experience of forgetting to bring a book,” which is a different meaning. Here we’re talking about a specific instance just now, so use 了 or nothing.

Any other natural alternatives for the first clause?

我刚才忘了带书。 (short, very common)
我刚才把书忘了。 (把 focus)
With place: 我刚才把书忘在公司了。
Negation contrast: 我刚才没带书 = “I didn’t bring the book (no claim about forgetting).”