Opettaja selittää hyvin, joten minä ymmärrän paremmin.

Yes, Finnish often does leave out subject pronouns when the verb ending already shows who the subject is.

So ymmärrän by itself already means I understand.

That means this sentence could also be:

Opettaja selittää hyvin, joten ymmärrän paremmin.

Including minä makes the subject more explicit. It can sound:

• a little more emphatic
• a little clearer for learners
• natural if you want contrast, like the teacher explains well, so I understand better

So minä is not wrong at all, but it is often optional here.

Finnish does not have articles like English a, an, and the.

So opettaja can mean:

• a teacher
• the teacher

You understand which one is meant from context.

In this sentence, opettaja is in the basic dictionary form, called the nominative singular, because it is the subject of the verb selittää.

Selittää is the 3rd person singular present tense form of the verb selittää, meaning to explain.

Here:

• opettaja = the subject
• selittää = explains / is explaining

So:

• Minä selitän = I explain
• sinä selität = you explain
• hän selittää = he/she explains
• opettaja selittää = the teacher explains

The same form is used for he, she, and nouns like opettaja.

Hyvin is an adverb meaning well.

It modifies the verb selittää, so selittää hyvin means explains well.

A useful comparison:

• hyvä = good
• hyvin = well

So:

• hyvä opettaja = a good teacher
• opettaja selittää hyvin = the teacher explains well

English also often distinguishes good and well, and Finnish does too.

Joten means something like:

• so
• therefore
• as a result

It connects two clauses, where the second is the result of the first.

Here:

• Opettaja selittää hyvin = the teacher explains well
• joten minä ymmärrän paremmin = so I understand better

So joten introduces a consequence.

It is different from koska, which means because:

• Ymmärrän paremmin, koska opettaja selittää hyvin.
  = I understand better because the teacher explains well.

So:

• koska gives the reason
• joten gives the result

In Finnish, a comma is normally used between two main clauses, and joten connects two main clauses here.

So:

• Opettaja selittää hyvin
• joten minä ymmärrän paremmin

Both parts could stand as their own sentence, so the comma is standard.

This is very natural Finnish punctuation.

Ymmärtää is the basic dictionary form, meaning to understand.

Ymmärrän is the 1st person singular present tense form, meaning I understand.

The ending -n often marks I in Finnish present tense verbs.

Compare:

• ymmärtää = to understand
• ymmärrän = I understand
• ymmärrät = you understand
• ymmärtää = he/she understands

This verb has a stem change:

• dictionary form: ymmärtää
• I form: ymmärrän

That change is normal for this verb.

Because this sentence needs an adverb, not an adjective.

• parempi = better, as an adjective
• paremmin = better, as an adverb

Here the word describes how I understand, so the adverb is needed:

• Ymmärrän paremmin = I understand better

Compare:

• Tämä kirja on parempi. = This book is better.
  -> adjective, describing book
• Nyt ymmärrän paremmin. = Now I understand better.
  -> adverb, describing understand

This is also slightly irregular:

• hyvä = good
• parempi = better
• hyvin = well
• paremmin = better

Yes, very naturally:

Opettaja selittää hyvin, joten ymmärrän paremmin.

This is probably the version you would hear very often in real Finnish.

Because ymmärrän already means I understand, the pronoun is not necessary unless you want:

• emphasis
• contrast
• extra clarity

So both are correct:

• ... joten minä ymmärrän paremmin
• ... joten ymmärrän paremmin

Yes, Finnish word order is more flexible than English, although the original sentence is very normal and neutral.

For example, these are possible depending on emphasis:

• Minä ymmärrän paremmin, koska opettaja selittää hyvin.
• Opettaja selittää hyvin, joten ymmärrän paremmin.

You can also move words for focus, but not every change sounds equally natural in every context.

The given sentence is a good neutral pattern:

• subject + verb + adverb
• connector
• subject + verb + adverb

So for learners, it is best to treat the original word order as the standard one.

No, they are different.

• paremmin = better
• enemmän = more

So:

• Ymmärrän paremmin = I understand better
• Ymmärrän enemmän = I understand more

These are close in meaning, but not identical.

Paremmin focuses on the quality of understanding.
Enemmän focuses on the amount of understanding.

In this sentence, paremmin fits very well because the teacher explains well, so the understanding improves.

Yes. Both verbs are in the present tense:

• selittää = explains / is explaining
• ymmärrän = I understand / I am understanding

Finnish present tense can cover both:

• habitual meaning: The teacher explains well, so I understand better
• current situation: The teacher is explaining well, so I understand better

Finnish does not usually make the same strong simple present vs. present continuous distinction that English does. Context tells you which reading is intended.