Sammuta nuo valot.

Sammuta is the singular imperative of sammuttaa.

That means it is the form used when you tell one person to do something:

• sammuttaa = to switch off / turn off / extinguish
• sammuta = turn off!

This is a very common pattern in Finnish. The dictionary form is often longer, while the command form is shorter:

• odottaa → odota
• auttaa → auta
• sammuttaa → sammuta

So Sammuta nuo valot. is a direct command to one person.

Because in Finnish, the subject is usually left out in commands.

So instead of saying:

• Sinä sammuta nuo valot

Finnish normally just says:

• Sammuta nuo valot

The verb form already shows that this is a command aimed at one person. Adding sinä would usually sound unusually emphatic, contrastive, or unnatural in an ordinary situation.

Nuo means those.

It is the plural form of tuo:

• tuo = that
• nuo = those

Finnish often makes a three-way distinction:

• tämä / nämä = this / these
• tuo / nuo = that / those
• se / ne = it/that/they/those, often very common in everyday speech

In this sentence, nuo valot means those lights, usually referring to specific lights that are visible or identifiable.

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

So Finnish does not need a separate word for the. Instead, definiteness is understood from context, or it can be made clearer with words like tämä, tuo, se, etc.

Here:

• nuo valot = those lights

In English, those already makes the noun definite, and Finnish works similarly here.

Valot is the plural form of valo:

• valo = light
• valot = lights

In this sentence, valot is the object of the command, and it refers to the lights as a whole, complete set: turn off those lights.

This is important because Finnish object forms change depending on meaning and sentence type. Here, valot expresses a total object: the action is meant to be completed.

Compare:

• Sammuta nuo valot. = Turn off those lights.
• Sammuta noita valoja. = Turn off some of those lights / turn those lights off in an incomplete or less bounded sense

For a beginner, the simplest way to remember it is: in this sentence, valot is the normal form for those lights as the thing being completely turned off.

Yes, in many situations you could hear those too, but the nuance and style are a little different.

• Sammuta nuo valot = standard Finnish, clear those lights
• Sammuta noi valot = colloquial spoken Finnish
• Sammuta ne valot = also very common in speech, often meaning those/the lights

A learner should know this:

• nuo is a good standard written form
• noi is its common spoken counterpart
• ne is extremely common in everyday speech and can overlap in meaning

So the sentence you were given is a very solid standard-Finnish version.

The verb comes first because this is a command, and that is the most neutral word order.

• Sammuta nuo valot. = neutral command

Finnish word order is fairly flexible, but changing it changes the emphasis:

• Nuo valot sammuta. = Those lights, turn them off.
  This sounds more marked or contrastive.

So for normal commands, verb first is the safest pattern.

It is addressed to one person.

That is because sammuta is the 2nd person singular imperative.

If you are speaking to more than one person, or using the plural/polite form, you would say:

• Sammuttakaa nuo valot.

Finnish can also use the plural form as a polite way to address one person in some contexts, though in modern everyday Finnish that kind of politeness is less central than in some other languages.

So:

• sammuta = command to one person
• sammuttakaa = command to several people / sometimes polite plural

A simple pronunciation guide:

• sammuta → SAM-mu-ta
• nuo → roughly nuo as one syllable, with uo as a diphthong
• valot → VA-lot

A few helpful points:

• Stress in Finnish is usually on the first syllable
• Double consonants are pronounced longer, so mm in sammuta is important
• Finnish spelling is very regular: you usually pronounce what you see

So the rhythm is roughly:

SAM-mu-ta NUO VA-lot

In Finnish, a command does not need an exclamation mark.

So:

• Sammuta nuo valot. = neutral written command
• Sammuta nuo valot! = stronger, more forceful, more emotional

A full stop is completely normal and often preferred in neutral writing.

No. Sammuttaa has a broader meaning than just turning off lights.

It can be used for things like:

• lights
• fires
• engines
• devices

So it often means turn off, switch off, or extinguish, depending on the object.

For example:

• sammuttaa valo = turn off a light
• sammuttaa tulipalo = put out a fire
• sammuttaa moottori = switch off the engine

So in Sammuta nuo valot, the verb fits very naturally with lights, but it is not limited to them.