Hän kertoo minulle, mitä opettaja sanoi.

Finnish does not have articles like the or a/an.

So opettaja can mean:

• a teacher
• the teacher

The context tells you which one is meant. In this sentence, English naturally uses the teacher, but Finnish just says opettaja.

In standard Finnish, hän is gender-neutral. It can refer to either a male or a female person.

So:

• Hän kertoo... = He tells... or She tells...

You only know which one is meant from context.

A native English speaker often expects separate words for he and she, but Finnish normally does not make that distinction in the pronoun.

Kertoo is the 3rd person singular present form of kertoa (to tell).

So:

• kertoa = to tell
• hän kertoo = he/she tells, is telling

In Finnish, the present tense often covers both:

• simple present: he/she tells
• present progressive: he/she is telling

The exact English translation depends on context.

Because the two verbs describe actions at different times.

• kertoo = the person is telling me now / in the current situation
• sanoi = what the teacher said happened earlier

So the structure is:

• He/She tells me now
• what the teacher said before

This is very natural in Finnish. The tense in the subordinate clause reflects the actual time of that action, not just the tense of the main verb.

Because kertoa usually marks the person receiving the information with -lle, which is the allative case.

So:

• minulle = to me
• sinulle = to you
• hänelle = to him/her

With kertoa, Finnish thinks in terms of telling something to someone.

Examples:

• Hän kertoo minulle. = He/She tells me.
• Hän kertoo ystävälleen uutisen. = He/She tells his/her friend the news.

So minulle is exactly what you expect after kertoa.

Here mitä means what and introduces the clause what the teacher said.

So:

• mitä opettaja sanoi = what the teacher said

This is not a direct quote. It is a subordinate clause that gives the content of what is being told.

A useful way to think of it:

• Hän kertoo minulle [mitä opettaja sanoi].

The bracketed part acts like the content of the telling.

Because mitä means what, while että means that.

Compare:

• Hän kertoo minulle, mitä opettaja sanoi.
  = He/She tells me what the teacher said.

• Hän kertoo minulle, että opettaja sanoi sen.
  = He/She tells me that the teacher said it.

In your sentence, the content itself is expressed as what the teacher said, so mitä is the correct choice.

Because Finnish normally uses a comma before a subordinate clause.

Here the subordinate clause is:

• mitä opettaja sanoi

So Finnish punctuation writes:

• Hän kertoo minulle, mitä opettaja sanoi.

This is a standard rule and is often more regular than in English.

The dictionary forms are:

• kertoa = to tell
• sanoa = to say

Then:

• kertoo = 3rd person singular present of kertoa
• sanoi = 3rd person singular past of sanoa

Very roughly:

• kertoa → kertoo
• sanoa → sanoi

So:

• hän kertoo = he/she tells
• hän sanoi = he/she said

These are very common verb patterns in Finnish, so they are worth learning early.

Yes, Finnish word order is fairly flexible, although the version you have is the most neutral and natural one.

Neutral order:

• Hän kertoo minulle, mitä opettaja sanoi.

Other orders are possible for emphasis, for example:

• Minulle hän kertoo, mitä opettaja sanoi.
  (emphasizes to me)

But the basic meaning stays the same. Finnish often changes word order to show emphasis, topic, or contrast rather than changing the core grammar.

Sometimes, yes. Finnish has no separate future tense.

So the present tense can refer to:

• present time
• near future
• scheduled future

But in this sentence, without extra context, kertoo is usually understood as tells / is telling.

If the context were future, it could also be understood as will tell.

Example:

• Huomenna hän kertoo minulle, mitä opettaja sanoi.
  = Tomorrow he/she will tell me what the teacher said.

It is very close to what many learners think of as an indirect question or an embedded what-clause.

In practical terms, you can treat it as a subordinate clause meaning:

• what the teacher said

The important point is that it is not direct speech. Compare:

• Opettaja sanoi: "..." = The teacher said: ...
• mitä opettaja sanoi = what the teacher said

So yes, it behaves like an embedded what-clause, which is often taught together with indirect questions.