Liike pysähtyi äkkiä.

Liike is the subject, pysähtyi is the verb, and äkkiä is an adverb modifying the verb.

So the basic pattern is:

• Liike = subject
• pysähtyi = verb
• äkkiä = adverb

This is a very normal Finnish sentence structure.

Because it is the subject of the sentence. In a normal affirmative sentence like this, the subject is usually in the nominative case, which is the dictionary/basic form for singular nouns.

So here:

• liike = nominative singular

Many Finnish nouns ending in -e use a different stem when case endings are added. With liike, the stem becomes liikke- in many forms.

For example:

• liike = nominative
• liikkeen = genitive
• liikettä = partitive

In this sentence, no extra case ending is needed, so you see the nominative form liike.

It is the 3rd person singular past tense of the verb pysähtyä.

So:

• pysähtyä = to stop, to come to a stop
• pysähtyi = stopped

More specifically, it means he/she/it stopped or came to a stop, depending on the subject.

Finnish dictionaries list verbs in the infinitive form, which here is pysähtyä. But in a sentence, you usually need a finite verb form that shows tense and person.

Here the verb stem is pysähty-, and the past tense uses -i-:

• pysähty-
  • i → pysähtyi

So pysähtyi is the past-tense sentence form, while pysähtyä is the dictionary form.

It is intransitive. That means the subject itself stops.

• Liike pysähtyi. = The movement stopped.

If you want to say that someone stopped something, Finnish normally uses pysäyttää, which is transitive:

• Hän pysäytti liikkeen. = He/She stopped the movement.

This is a very useful distinction:

• pysähtyä = to stop oneself / come to a stop
• pysäyttää = to stop something

Finnish has no articles, so there is no separate word for a/an or the.

That means liike can mean things like:

• movement
• a movement
• the movement

The exact English choice depends on context. In your translation, context tells you which one is meant.

Many Finnish adverbs do end in -sti, but not all of them. Äkkiä is a very common adverb in its own right.

Here it means suddenly. It tells you how the action happened.

So:

• pysähtyi = stopped
• pysähtyi äkkiä = stopped suddenly

A learner should not expect every Finnish adverb to have a visible -sti ending.

Yes. Finnish word order is fairly flexible, although some orders sound more neutral than others.

Liike pysähtyi äkkiä is a neutral, standard order.

If you change the order, the meaning stays close, but the focus may change. For example:

• Äkkiä liike pysähtyi.

This gives more emphasis to äkkiä, like Suddenly, the movement stopped.

So Finnish word order is not as rigid as English word order.

No, because the subject noun liike is already there.

Finnish does not add an extra pronoun when the full subject noun is stated. So:

• Liike pysähtyi = correct
• adding a separate it-type word would be unnecessary

If the subject were only a pronoun, Finnish would normally use one, for example:

• Se pysähtyi. = It stopped.

But in your sentence, liike already does the job.

A rough pronunciation guide is:

• Liike → LII-ke
• pysähtyi → PY-säh-tyi
• äkkiä → ÄK-ki-ä

A few helpful points:

• Finnish stress is normally on the first syllable.
• ii in liike is a long vowel.
• kk in äkkiä is a long/double consonant, so it is held slightly longer.
• ä is pronounced somewhat like the vowel in English cat, but cleaner and more consistent.
• y is a front rounded vowel, similar to German ü or French u.

So the rhythm is quite regular: LII-ke PY-säh-tyi ÄK-ki-ä.