Ymmärrän helpommin, kun opettaja puhuu hitaasti.

Because Finnish usually leaves out the subject pronoun when the verb ending already shows who is doing the action.

• ymmärrän = I understand
• the ending -n tells you it is first person singular

So Ymmärrän helpommin... already clearly means I understand more easily...
You can add minä for emphasis or contrast:

• Minä ymmärrän, mutta hän ei. = I understand, but he/she doesn’t.

Ymmärrän is the present tense, first person singular form of the verb ymmärtää = to understand.

So:

• minä ymmärrän = I understand

In this sentence, the present tense expresses a general situation or habit, not just something happening right now.

Because helpommin is an adverb, and here it modifies the verb ymmärrän.

• helpompi = easier (adjective)
• helpommin = more easily (adverb)

English sometimes says I understand easier, but more standard English would be I understand more easily. Finnish clearly uses the adverb form here.

They are related, but not the same.

• helposti = easily
• helpommin = more easily

So helpommin is the comparative form. It suggests a comparison, even if the other side of the comparison is not stated directly.

• Ymmärrän helposti. = I understand easily.
• Ymmärrän helpommin, kun... = I understand more easily when...

The sentence implies: compared with other situations, I understand more easily when the teacher speaks slowly.

Here kun means when.

It introduces a subordinate clause that gives the circumstance in which the main clause happens:

• kun opettaja puhuu hitaasti = when the teacher speaks slowly

Why not the others?

• jos = if
  This would make the sentence more conditional:

  • Ymmärrän helpommin, jos opettaja puhuu hitaasti.
  • I understand more easily if the teacher speaks slowly.

• koska = because
  This would make the meaning causal:

  • Ymmärrän helpommin, koska opettaja puhuu hitaasti.
  • I understand more easily because the teacher speaks slowly.

In many contexts, kun is the most natural choice for when.

Because the subject is opettaja = teacher, which is third person singular.

Finnish verbs agree with the subject:

• minä puhun = I speak
• sinä puhut = you speak
• hän/opettaja puhuu = he/she/the teacher speaks

So:

• opettaja puhuu = the teacher speaks

Because hitaasti is an adverb, and it tells you how the teacher speaks.

• hidas = slow (adjective)
• hitaasti = slowly (adverb)

Since it modifies the verb puhuu, you need the adverb:

• opettaja puhuu hitaasti = the teacher speaks slowly

It comes from the adjective hidas = slow.

A very common way to form adverbs in Finnish is with -sti:

• hidas → hitaasti = slowly
• varma → varmasti = certainly
• nopea → nopeasti = quickly

So this is a useful pattern to learn: adjectives often have a corresponding -sti adverb.

Finnish has no articles. There is no direct equivalent of a/an/the in normal grammar.

So opettaja can mean:

• a teacher
• the teacher

The exact meaning depends on context. In this sentence, English naturally uses the teacher, but Finnish does not need a separate word for that.

Yes, in standard written Finnish, a comma is normally used between the main clause and the subordinate clause.

So:

• Ymmärrän helpommin, kun opettaja puhuu hitaasti.

That comma helps show the sentence structure:

• main clause: Ymmärrän helpommin
• subordinate clause: kun opettaja puhuu hitaasti

Yes. You can say:

• Kun opettaja puhuu hitaasti, ymmärrän helpommin.

This means the same thing. The difference is mainly in emphasis and flow.

• Starting with Ymmärrän helpommin... puts the focus first on I understand more easily
• Starting with Kun opettaja puhuu hitaasti... puts the situation first: when the teacher speaks slowly

Both are natural.

Because Finnish can leave the object unstated when it is obvious from context.

Here Ymmärrän helpommin means something like:

• I understand more easily
• I understand what is being said more easily
• I understand the teacher’s speech more easily

The sentence does not need to state exactly what is understood, because the listener can infer it from the situation. This is normal in Finnish.