Älä päästä koiraa ulos.

Älä is the negative imperative for you (singular). In other words, it means don’t when you are telling one person not to do something.

So:

• Älä mene. = Don’t go.
• Älä puhu. = Don’t speak.
• Älä päästä koiraa ulos. = Don’t let the dog out.

It is specifically used when speaking to one person. If you were speaking to more than one person, Finnish would use a different form.

The basic dictionary form of the verb is päästää, which means to let, to release, or to allow to go.

After älä, Finnish uses a special verb form called the connegative, and for this verb that form is päästä.

So:

• dictionary form: päästää
• after älä: päästä

That is why the sentence is:

• Älä päästä koiraa ulos.

and not:

• Älä päästää koiraa ulos. ❌

This pattern is very common in Finnish negative commands.

Because the sentence is negative, the object usually goes into the partitive case in Finnish.

Here:

• koira = dog (basic form)
• koiraa = dog in the partitive singular

So in a negative sentence:

• Älä päästä koiraa ulos.

the dog is the object of the action, and because the command is negative, Finnish uses koiraa.

A useful comparison:

• Päästä koira ulos. = Let the dog out.
• Älä päästä koiraa ulos. = Don’t let the dog out.

So this is a very important rule:
negation often triggers the partitive object.

Ulos means out or outward, with a sense of movement from inside to outside.

So it is not just a general word for outside. It specifically suggests going out.

Compare:

• ulos = out / to the outside
• ulkona = outside in the sense of being outside
• ulkona answers where?
• ulos answers to where?

Examples:

• Koira on ulkona. = The dog is outside.
• Koira menee ulos. = The dog goes outside.

In your sentence, ulos is used because the idea is letting the dog go out.

Often yes, but not always perfectly.

In this sentence, päästää works very much like let:

• päästää koira ulos = let the dog out

But päästää can also have meanings like:

• release
• allow to leave
• set free
• let something happen

So the core idea is often something like allowing or causing someone/something to get free, go somewhere, or be released.

In this sentence, the most natural English translation is definitely let the dog out.

It is a command to one person.

That is shown by älä, which is the singular don’t command.

If you were speaking to several people, Finnish would use a different form, for example:

• Älkää päästäkö koiraa ulos. = Don’t let the dog out. (said to several people)

So the original sentence is directed at you singular.

Yes, depending on context, it could.

The verb päästää has a range of meanings related to letting go, releasing, or allowing out. In everyday home situations, with koira and ulos, the natural meaning is:

• Don’t let the dog out.

But in a different context, there could be a stronger sense of release. The word ulos still strongly points toward out/outside, though, so the everyday interpretation is the most likely one.

A rough pronunciation guide for an English speaker would be:

• Älä ≈ EH-lah
• päästä ≈ PAES-tah
• koiraa ≈ KOY-raa
• ulos ≈ OO-los

A few important points:

• ä is not like English a in cat, but it is a front vowel somewhat similar to the vowel in many pronunciations of bed or bad, depending on accent.
• ää is a long vowel, so hold it a little longer.
• oi in koiraa glides somewhat like oy.
• Finnish pronunciation is usually quite regular, and stress is normally on the first syllable:
  • Ä-lä
  • PÄÄS-tä
  • KOI-raa
  • U-los

Finnish does not have articles like a, an, or the.

So koira can mean:

• a dog
• the dog

Which one is meant depends on context.

In this sentence, the meaning is naturally understood as the dog, because in English that is how we would usually say it:

• Don’t let the dog out.

But Finnish itself does not add a separate word for the.

The structure is:

• Älä = negative command word
• päästä = main verb in the connegative form
• koiraa = object in the partitive
• ulos = direction, out

So the sentence pattern is basically:

Don’t + let + dog + out

More literally:

Do not let dog out

Finnish word order is fairly natural here and matches the meaning clearly. The most neutral order is exactly the one you see:

• Älä päästä koiraa ulos.

The positive command would be:

• Päästä koira ulos. = Let the dog out.

Notice two changes:

1. Älä disappears.
2. koiraa becomes koira.

That second change happens because the object is no longer under negation. So this pair is very useful to compare:

• Päästä koira ulos. = Let the dog out.
• Älä päästä koiraa ulos. = Don’t let the dog out.

This is a great example of how negation affects the object case in Finnish.