Learn Hardcore Chinese: Tā jīntiān yòu wàngjì dài shǒujī, yòu chídào le. - Today she both forgot her phone and was late.

又 marks a repetition that has already happened by the reference time (past or “already happened today”). It often carries a complaining tone.
再 is used for future/planned/hypothetical repetition.
Examples:
- 她今天又迟到了。= She was late again today. (already happened)
- 她明天再迟到就糟了。= If she’s late again tomorrow, that’ll be bad. (future)

Why are there two 又’s? Do I need both?

Each 又 scopes over the verb phrase that follows it.
With two: both actions are “again” (forgot the phone again, was late again).
If you drop the second: 她今天又忘记带手机，迟到了。 Only the first action is explicitly “again”; the second is just a fact (not necessarily again).

Can I replace the second 又 with 也 or 还? What’s the difference?

也 = also/too; adds another fact but doesn’t mean “again.”
还 = in addition/on top of that; often adds a mildly complaining “piling on” tone; still not “again.”
Compare:
- 她今天又忘记带手机，也迟到了。 (phone: again; late: also)
- 她今天又忘记带手机，还迟到了。 (phone: again; late: on top of that)
- 她今天又忘记带手机，又迟到了。 (both repeated)

Is this the 又A又B “both … and …” pattern?

Not here. The “又A又B” pattern typically links adjectives/stative predicates (e.g., 他又高又帅).
In your sentence, 又 is the adverb “again” repeated before two separate events.

What does 了 do in 又迟到了?

It’s the verb-suffix 了 marking the completion/occurrence of “being late.”
It’s not necessarily the sentence-final 了 (change-of-state), though it happens to be at the end of the sentence.
With 今天, you could omit 了 and be understood, but 迟到了 sounds more natural and event-like than just 迟到.

Can I put 了 after 忘记 instead? (e.g., 她今天又忘记带手机了)

Yes: 她今天又忘记带手机了，又迟到了。 is fine; here the first 了 marks the completion of “forgetting to bring.”
Often speakers put 了 only once after the last predicate to avoid repetition: 她今天又忘记带手机，又迟到了。
If you want to highlight the first event as new info and then add another fact, use: 她今天又忘记带手机了，还迟到了。

What’s the correct word order for 今天 and 又?

Why use 带 for 手机? Can I use 拿 or 带上?

带 = bring/carry with you (over a period) → default for “bring your phone.”
拿 = take/hold (the physical act of picking up); used when focusing on the hand action or location: 出门别忘了拿手机（从桌子上）.
带上 = put on/take along (at the moment of departure), slightly more dynamic: 出门别忘了带上手机.
After the fact, “忘记带手机” is the most idiomatic.

Why “忘记带手机” and not “忘记手机”?

忘记手机 literally means “forget the phone (exists),” which doesn’t express “forget to bring.”
For “forget to do X,” use 忘记 + Verb: 忘记带手机.
Alternative: 把-pattern: 她又把手机忘在家里了。= She forgot (left) her phone at home again.

Are “忘带手机” and “忘了带手机” also correct?

Where should 了 go with 忘(记)? Is “忘记了带手机” OK?

Natural placements:
- 忘了带手机
- 忘记带手机了
“忘记了带手机” (putting 了 inside “忘记 + 带手机”) is awkward. Don’t split the verb and its complement that way.

Do I need a measure word before 手机?

Could I connect the clauses with 而且?

It’s possible but not the best fit here. 而且 often pairs with 不但/不仅 and doesn’t express “again.”
“她今天又忘记带手机，而且又迟到了” is grammatical but redundant and less natural than using 又/还/也 as shown above.

Is 又 mainly for complaints or negative events?

Is 迟到 a verb? Do I need an object?

How do I talk about “again” in the future?

Use 再:
- 她明天再迟到就糟了。= If she’s late again tomorrow, that’s bad.
- 别再迟到了。= Don’t be late again.
- 别再忘记带手机了。= Don’t forget to bring your phone again.

Any other natural rephrasings?

Pronunciation check: 又 vs 有?