A billentyűzetet a képernyő mellé teszem.

Because billentyűzet is the direct object of the verb, so it takes the accusative ending.

• billentyűzet = keyboard
• billentyűzetet = keyboard (as the thing being put somewhere)

In Hungarian, the direct object usually gets -t, but in many words an extra vowel appears before it:

• könyv → könyvet
• asztal → asztalt
• billentyűzet → billentyűzetet

So here, a billentyűzetet means the keyboard as the object of teszem = I put.

Because Hungarian verbs can have two kinds of conjugation:

• indefinite conjugation: used when there is no definite object
• definite conjugation: used when the object is definite

Here the object is a billentyűzetet = the keyboard, which is definite, so the verb uses the definite form:

• teszek = I put / I place (something indefinite)
• teszem = I put it / I place the ... (something definite)

So:

• Billentyűzetet teszek... would sound like I’m putting a keyboard...
• A billentyűzetet teszem... means I’m putting the keyboard...

Because a is the Hungarian definite article, meaning the.

• a billentyűzetet = the keyboard
• a képernyő = the screen

Hungarian uses a or az depending on the sound that follows:

• a before most consonant sounds
• az before vowel sounds

Examples:

• a billentyűzet
• a képernyő
• az asztal

So both nouns are definite in this sentence.

Mellé means to beside, to next to, or to the side of. It expresses movement toward a position next to something.

So:

• a képernyő mellé = next to the screen / to beside the screen

This is different from mellett, which describes a location rather than motion:

• a képernyő mellett = next to the screen (already there)
• a képernyő mellé = to next to the screen (moving it there)

Because teszem involves placing something somewhere, mellé is the right choice.

Because képernyő is not the direct object here. The keyboard is the thing being moved, so billentyűzetet gets the accusative ending.

The screen is only the reference point: the keyboard is being placed next to the screen. That relationship is shown by mellé, not by the accusative.

So the structure is:

• a billentyűzetet = the keyboard (object)
• a képernyő mellé = next to the screen (destination/location phrase)

Only the direct object takes the accusative here.

Because Hungarian usually does not need subject pronouns when the verb ending already shows who is doing the action.

• teszem already means I put
• so én teszem is usually unnecessary

Hungarian often omits pronouns unless they are needed for emphasis or contrast.

For example:

• A billentyűzetet a képernyő mellé teszem. = I put the keyboard next to the screen.
• Én teszem a billentyűzetet a képernyő mellé. = I’m the one putting the keyboard next to the screen.

So the version without én is the normal one.

Yes, Hungarian word order is flexible, and it often reflects topic, focus, and emphasis, not just grammar.

In this sentence:

• A billentyűzetet comes first, so it is the topic: we are talking about the keyboard.
• Then comes a képernyő mellé
• Then teszem

This can feel like:

• As for the keyboard, I put it next to the screen.

A different word order can shift emphasis:

• A billentyűzetet a képernyő mellé teszem.
  The keyboard is the main thing being talked about.

• A képernyő mellé teszem a billentyűzetet.
  This can sound a bit more like the location is being highlighted.

• A billentyűzetet teszem a képernyő mellé.
  This can put stronger focus on the keyboard as opposed to something else.

So the sentence is natural, but the order is not random.

The dictionary form is tenni, which means to put, to place, or sometimes to do in certain expressions.

Some forms are:

• tenni = to put
• teszek = I put (indefinite)
• teszem = I put it / I put the... (definite)
• tesz = he/she puts
• teszi = he/she puts it

So teszem comes from the verb tenni.

In this sentence, mellé is a separate postposition-like form meaning to beside / next to.

Hungarian often expresses spatial relationships with:

• case endings
• postpositions
• or forms closely related to both kinds of patterns

For a learner, the important practical point is this:

• a képernyő mellé functions as one unit meaning next to the screen / to the side of the screen

You do not attach mellé directly to képernyő in this sentence. It stays separate:

• a képernyő mellé

Yes. Képernyő literally means screen, but in context it can often be translated naturally as monitor.

So depending on the situation, English might say:

• I put the keyboard next to the screen.
• I’m putting the keyboard next to the monitor.

Both can be reasonable translations. Hungarian uses képernyő, and English chooses the most natural word for the context.

Both can mean I put it next to something, but the verb changes slightly in tone and common usage.

• teszem comes from tenni = to put/place
• rakom comes from rakni = to put/set/place

In many everyday situations, both are possible. Rakni can sometimes sound a little more physical or colloquial, while tenni is very general.

So:

• A billentyűzetet a képernyő mellé teszem.
• A billentyűzetet a képernyő mellé rakom.

Both are normal, with only a small difference in feel.