Olin vähällä kompastua pimeydessä.

It uses the Finnish pattern olla vähällä + infinitive, which means to be about to do something or to almost do something.

So:

• olin = I was
• vähällä = part of this fixed expression
• kompastua = to trip

Together, olin vähällä kompastua means I almost tripped / I was about to trip.

This is a very common Finnish structure.

Olin is the past tense form of olla for I.

• olen = I am
• olin = I was

Because the sentence describes something that happened in the past, Finnish uses olin.

Because the expression olla vähällä is followed by the first infinitive.

So the structure is:

• olin vähällä tehdä
• olit vähällä kaatua
• hän oli vähällä myöhästyä

In this sentence:

• kompastua = to trip

It stays in the infinitive because it is the action that was almost happening.

Vähällä is the adessive form of vähä/vähä-, but in this sentence it works as part of a fixed idiomatic expression: olla vähällä.

You do not usually translate it literally word-for-word. It is best learned as a whole unit:

• olla vähällä tehdä jotain = to almost do something

So even though vähällä may look strange on its own, here it is just part of a set phrase.

Kompastua means to trip or to stumble.

It is normally intransitive, meaning it does not take a direct object in the way English sometimes does.

For example:

• Kompastuin kiveen. = I tripped over a stone.

Notice that Finnish uses something like to a stone / on a stone / over a stone rather than making stone a direct object.

So in your sentence, kompastua simply describes the action of tripping.

Pimeydessä is in the inessive case, which often means in something.

The base form is:

• pimeys = darkness

Then:

• pimeydessä = in the darkness / in darkness

So the sentence says that the near-tripping happened in the dark.

Both can be possible, but they are slightly different in feel.

• pimeässä = in the dark
  
  • very common and everyday
  • based on pimeä = dark
• pimeydessä = in darkness / in the darkness
  • based on pimeys = darkness
  • can sound a little more abstract, literary, or formal

So:

• Olin vähällä kompastua pimeässä. = a very natural everyday way to say it
• Olin vähällä kompastua pimeydessä. = also correct, but a bit more like in darkness

Finnish often leaves out subject pronouns when the verb already shows who the subject is.

Here:

• olin already clearly means I was

So minä is unnecessary unless you want extra emphasis.

Compare:

• Olin vähällä kompastua pimeydessä. = normal
• Minä olin vähällä kompastua pimeydessä. = more emphatic, like I was the one who almost tripped

Yes. Finnish word order is fairly flexible, and changing it often changes the emphasis, not the basic meaning.

For example:

• Olin vähällä kompastua pimeydessä. = neutral
• Pimeydessä olin vähällä kompastua. = emphasizes in the darkness
• Kompastua olin vähällä pimeydessä. = possible in special contexts, but less neutral

The most natural basic order here is the original one.

You would negate olla in the usual Finnish way:

• En ollut vähällä kompastua pimeydessä.

That means:

• I was not about to trip in the dark
• or, depending on context, I wasn’t close to tripping in the dark

So the pattern stays the same:

• en ollut / et ollut / ei ollut + vähällä + infinitive

They are close, but not always identical.

• Olin vähällä kompastua.
• Melkein kompastuin.

Both can mean I almost tripped.

But the structures are different:

• olla vähällä + infinitive
• melkein + finite verb

Examples:

• Olin vähällä unohtaa avaimet.
• Melkein unohdin avaimet.

Both are natural.
Olla vähällä can sometimes feel a little more like I was on the verge of doing it, while melkein is the more general word for almost.