Я нажал не ту клавишу на пианино.

Russian has no articles, so it does not use separate words for a or the.

In Я нажал не ту клавишу на пианино, definiteness is understood from context, and ту helps a lot. Ту means that and here it strongly suggests the one I meant / the intended one. So English may use the wrong key, while Russian naturally says не ту клавишу.

Нажал is the perfective past tense of нажать, and it means a single completed action: the speaker pressed the key.

That fits this sentence well, because the idea is that one specific mistake happened: I pressed the wrong key.

Нажимал is imperfective and would suggest:

• a repeated action,
• an ongoing action,
• or focus on the process rather than the completed result.

So:

• Я нажал не ту клавишу = I pressed the wrong key.
• Я нажимал не ту клавишу would sound more like I was pressing the wrong key repeatedly / I kept pressing the wrong key / I was in the process of pressing it.

That is the normal marker of the past tense in Russian.

In the singular, past tense verbs also show gender:

• нажал = masculine
• нажала = feminine
• нажало = neuter

So this sentence suggests that the speaker is male. If the speaker were female, it would be:

Я нажала не ту клавишу на пианино.

Because ту is the accusative feminine singular form of та.

The noun клавиша is feminine, and here it is the direct object of нажал. Direct objects often go into the accusative case.

So:

• та клавиша = that key, in the nominative
• ту клавишу = that key, in the accusative

This is a case-and-agreement issue:

• та matches nominative feminine singular
• ту matches accusative feminine singular

For the same reason: клавишу is the accusative singular of клавиша.

Since the key is what was pressed, it is the direct object.

So:

• клавиша = nominative, the subject form
• клавишу = accusative, the object form

Compare:

• Клавиша сломалась. = The key broke.
  Here клавиша is the subject.
• Я нажал клавишу. = I pressed the key.
  Here клавишу is the object.

Literally, не ту means not that one.

In natural English, though, this is usually translated as:

• the wrong one
• the wrong key
• not the key I meant

So Я нажал не ту клавишу is a very natural Russian way to say I pressed the wrong key.

Russian often uses не тот / не та / не то / не те in this way:

• Я выбрал не тот файл. = I chose the wrong file.
• Мы сели не в тот поезд. = We got on the wrong train.

You can say неправильную клавишу, but it is usually less natural in this situation.

Не ту клавишу means:

• not the one intended,
• the wrong one by mistake.

That is exactly the idea here.

Неправильную клавишу sounds more like:

• an incorrect key according to some rule,
• the wrong key in a more abstract or technical sense.

In everyday Russian, when someone simply hit the wrong button/key/object, не ту is often the most natural choice.

No. Клавиша can mean a key or button on various devices or instruments.

For example, it can refer to:

• a piano key,
• a computer keyboard key,
• a button-like key on a machine or instrument.

In this sentence, на пианино makes it clear that it means a piano key.

Important contrast:

• клавиша = key/button on an instrument or device
• ключ = a key for a lock

Because на here means on.

The idea is that the key is on the piano, so Russian says:

• на пианино = on the piano

This is the natural prepositional phrase with пианино in this context.

Using в пианино would mean something more like inside the piano, which is not what is meant here.

Because пианино is an indeclinable noun in Russian.

That means it keeps the same form in all cases:

• пианино
• на пианино
• у пианино
• без пианино

Many borrowed words in Russian behave this way.

Grammatically, пианино is usually treated as neuter, even though its form does not change.

Yes, very often.

Russian often omits the subject pronoun when it is clear from context. So in conversation, someone might simply say:

Нажал не ту клавишу на пианино.

That still means I pressed the wrong key on the piano, if the context makes it obvious who is speaking.

Including Я can sound:

• neutral,
• slightly more explicit,
• or more contrastive, depending on context.

Yes. Russian word order is flexible, though different orders change the emphasis.

The neutral version is:

Я нажал не ту клавишу на пианино.

But you could also hear:

Я не ту клавишу нажал на пианино.
This puts more emphasis on the wrong key.

Or:

На пианино я нажал не ту клавишу.
This can emphasize on the piano or set the scene first.

The basic meaning stays the same, but the focus shifts.

The main word stresses are:

Я нажáл не ту клáвишу на пианíно.

So:

• нажáл
• клáвишу
• пианíно

Stress is important in Russian, because it is not always predictable and can affect pronunciation a lot.