Learn Hardcore Japanese: sensei ha gakusei ni, maikai kyoukasyo wo wasurenai you ni iimasita. - The teacher told the students not to forget their textbook each time.

先生は: introduces the teacher as the one we’re talking about.
先生が: would mark the teacher more strongly as the grammatical subject, often used when identifying who did something, in contrast to someone else.

Here, the main point is what the teacher said to the students, not contrasting the teacher with others, so は is natural as the topic marker.

What does に after 学生 mean here?

The に after 学生 marks the indirect object / target person of the action 言いました (“said (to)”).

学生に言いました = “(the teacher) said (something) to the students.”

This is the standard pattern:

A は B に言う = “A says (something) to B.”
- 先生は学生に言いました。
- 友だちは私に言いました。 (“My friend told me.”)

What does 毎回 mean, and why is it before 教科書?

毎回 (まいかい) means “every time”.

Placed before 教科書を忘れないように, it modifies the action “not forget the textbook”:

毎回教科書を忘れないように
→ “so that (they) don’t forget the textbook every time.”

Word order is quite flexible, but this position is natural:

Time expression (毎回)
Object (教科書を)
Verb phrase (忘れないように)

You could also say 教科書を毎回忘れないように, but that sounds a little less smooth and more marked. The original order is more neutral.

What is the meaning and function of 忘れないように?

忘れないように comes from:

忘れない = “do not forget” (negative, non‑past of 忘れる)
ように = “so that / in such a way that”

Together: 忘れないように means “so that (you/they) don’t forget” or “in order not to forget.”

This expression describes a desired state or outcome, not an action the speaker is directly performing. It’s often used with verbs like:

覚える → 覚えるように (“so that (you) remember”)
遅れる → 遅れないように (“so that (you) don’t be late”)

Why is it 忘れないように言いました and not something like 忘れてはいけないと言いました or a direct command?

All of these are possible, but the nuance changes:

忘れないように言いました
- Literally: “(The teacher) said (to them) so that they don’t forget.”
- Nuance: gentle instruction / request, emphasizing the desired outcome (“Please make sure you don’t forget.”).
忘れてはいけないと言いました
- “(The teacher) said (that) they must not forget.”
- Stronger, more rule-like: “You must not forget.”
Direct command, e.g. 忘れるな！
- “Don’t forget!”
- Very direct, can sound harsh or very strong.

Using Vないように言う is a common, relatively polite and soft way to report instructions:
〜ないように言いました = “told (someone) to not do ~ / to make sure not to ~.”

Why is the verb inside ように in the negative form (忘れない) instead of positive?

In English we say “don’t forget”, which already uses a negative. Japanese does the same inside the 〜ように pattern:

忘れないように = “so that (you) don’t forget”

If you used the positive:

忘れるように = “so that (you) forget”

That would mean the opposite: encouraging forgetting, which is not intended here. So the negative form expresses the undesired action we want to avoid.

What does ように + 言いました express as a pattern?

The pattern is:

(Person) は (Target) に V-る / V-ないように言う

It means:

“(Person) told (Target) to do / not do V”
with a nuance of request / instruction, often softer than a direct order.

Examples:

先生は学生にもっと勉強するように言いました。
→ “The teacher told the students to study more.”
母は私に夜更かししないように言いました。
→ “My mother told me not to stay up late.”

So in the given sentence, 忘れないように言いました = “told (them) not to forget.”

What is the role of を after 教科書 when the verb is 忘れない (negative)?

を marks 教科書 as the direct object of the verb 忘れる (“to forget”), even though the verb is negated:

(教科書を) 忘れる = to forget (the textbook)
(教科書を) 忘れない = to not forget (the textbook)

The object marker を is used the same way whether the verb is positive or negative; the negation just applies to the action itself.

How does the past tense 言いました work with the future-like idea of “not forgetting every time”?

言いました is past polite form of 言う: “said / told.” It locates the act of saying in the past.

However, inside the ように clause, 忘れない is non‑past. Japanese non‑past covers both present and future. So the time relationship is:

In the past, the teacher told them
(At that time) about a present/future habit or action: “not forgetting the textbook every time.”

This is normal in Japanese:

先生は来週遅れないように言いました。
→ “The teacher told (us) not to be late next week.”
(“Told” is past; “be late” is future relative to that.)

Could the word order be changed, for example putting 毎回 or 学生に in different places?

Japanese word order is somewhat flexible, especially for modifiers and adverbials. Some variations:

先生は学生に毎回教科書を忘れないように言いました。
先生は学生に教科書を毎回忘れないように言いました。 (a bit less natural)
先生は毎回学生に教科書を忘れないように言いました。 (focuses more on the teacher saying it every time)

The original:

先生は学生に、毎回教科書を忘れないように言いました。

is clear and natural:
topic (先生は) → target person (学生に) → time-related adverb (毎回) → object (教科書を) → verb phrase.

What is the politeness level of 言いました, and how would it change with 言った?

言いました is the polite past form (ます‑form). Using it makes the narration polite, suitable for talking to someone you address respectfully (like in a textbook or formal conversation).

言いました → polite: “said / told”
言った → plain: “said / told”

If we change just that part:

先生は学生に、毎回教科書を忘れないように言いました。
→ polite narration.
先生は学生に、毎回教科書を忘れないように言った。
→ plain narration, used in casual writing, stories, or among people of equal status.

The meaning is the same; only the speech level changes.