Olin vähällä painaa väärää painiketta.

Olla vähällä + infinitive is a common Finnish expression meaning to be about to do something or to almost do something.

So:

Olin vähällä painaa väärää painiketta
= I almost pressed the wrong button
= I was about to press the wrong button

It is best learned as a set expression.

Usually no.

That is one important thing about olla vähällä + infinitive: it normally means the action nearly happened, but did not happen.

So this sentence suggests:

• the speaker came very close to pressing the wrong button
• but stopped in time

Olin is the 1st person singular past tense of olla, so it means I was.

Basic forms:

• olen = I am
• olin = I was

Because the situation happened in the past, Finnish uses olin.

Compare:

• Olen vähällä painaa väärää painiketta = I am about to press the wrong button
• Olin vähällä painaa väärää painiketta = I was about to / almost pressed the wrong button

Because after olla vähällä, the next verb is in the first infinitive form.

Here:

• painaa = to press

So the structure is:

• olin = I was
• vähällä = on the verge of / close to
• painaa = to press

This is similar to how English uses to press after was about to.

Here the object is in the partitive: painiketta.

With the verb painaa meaning to press, Finnish very often uses the partitive when talking about pressing a button. In everyday Finnish, painaa nappia / painiketta is very natural.

So:

• väärää painiketta = the wrong button, in the partitive form

A simple way to understand it is:

• Finnish is focusing on the act of pressing
• not on a fully completed result affecting the whole object

For a learner, the safest thing is to remember that painaa painiketta / nappia is a very common pattern.

Because adjectives in Finnish usually agree with the noun they describe.

The basic forms are:

• väärä = wrong
• painike = button

But in the sentence, the noun is painiketta in the partitive singular, so the adjective must match it:

• väärää painiketta

So both words show the same case.

They are very close in meaning.

• painike = button, often a bit more technical or formal
• nappi = button, more everyday and also used for clothing buttons

In many situations, either word works:

• painaa painiketta
• painaa nappia

Both can mean press a button.

So this sentence could also naturally be said as:

Olin vähällä painaa väärää nappia.

Yes, absolutely.

Melkein painoin väärää painiketta also means I almost pressed the wrong button.

The difference is mostly in style and nuance:

• Olin vähällä painaa... = I was on the verge of pressing...
• Melkein painoin... = I almost pressed...

Both are common.
Olla vähällä can sound a bit more like the action was very close to happening.

This is the normal, neutral word order.

• Olin = I was
• vähällä painaa = about to press
• väärää painiketta = the wrong button

Finnish word order is fairly flexible, but this version is the most natural basic way to say it.

A different word order could be used for emphasis, but learners should treat this sentence as the standard pattern.