Laitan kylmää mehua lasiin, kun ilma on lämmin.

In Finnish, the subject pronoun is often left out when the verb ending already shows who is doing the action.

• laitan = I put / I pour
• the ending -n tells you it is first person singular

So Laitan... already means I put..., and adding minä would usually only be for emphasis.

Yes, laittaa basically means to put / place / set, but in everyday Finnish it is also used quite broadly.

In this sentence, Laitan kylmää mehua lasiin means something like:

• I put cold juice into a glass
• or more naturally in English, I pour cold juice into a glass

A more specific verb for liquids would often be kaataa (to pour), so Kaadan kylmää mehua lasiin is also very natural. But laitan is still grammatical and understandable.

They are in the partitive because the sentence refers to an unspecified amount of juice, not a whole, clearly bounded object.

Finnish often uses the partitive for:

• mass nouns like juice, water, coffee
• some amount of something
• actions that do not focus on a complete finished object

So:

• mehua = (some) juice
• kylmää mehua = (some) cold juice

If you were talking about a whole specific item, Finnish might use a different object form.

Because adjectives usually agree with the nouns they describe in case and number.

Here:

• base form: kylmä mehu
• partitive singular: kylmää mehua

So both words take the partitive singular ending pattern.

This is very normal in Finnish:

• isoa taloa
• kaunista kukkaa
• kylmää mehua

Because lasiin is the illative case, which often means into.

So:

• lasi = glass
• lasiin = into a glass

The sentence is describing movement or direction: the juice goes into the glass.

This is one of the common Finnish location/direction contrasts:

• lasissa = in the glass
• lasista = out of the glass / from the glass
• lasiin = into the glass

The basic word is lasi. Its illative form is lasiin.

With many words of this type, the illative singular is made by adding:

• a long vowel
• plus -n

So:

• lasi → lasiin
• koti → kotiin
• joki → jokeen (not all words look exactly the same, but the idea is similar)

It is best to learn illative forms gradually by exposure, because Finnish has a few patterns.

Here kun means when.

So the sentence means that this happens when the weather is warm.

Difference:

• kun = when
• jos = if

Examples:

• Kun ilma on lämmin, laitan kylmää mehua lasiin. = When the weather is warm, I put/pour cold juice into a glass.
• Jos ilma on lämmin, laitan kylmää mehua lasiin. = If the weather is warm, I’ll put/pour cold juice into a glass.

Kun sounds more like a real or habitual situation; jos sounds more conditional.

Yes, ilma can mean air, but in many contexts it also means weather / the air outside / the atmosphere.

In ilma on lämmin, it is usually understood as:

• the weather is warm
• or the air is warm

Both ideas are close, and English often chooses the weather in translation.

A related word is sää, which more directly means weather.
So sää on lämmin is not the normal expression, but sää on lämmin actually sounds odd because lämmin describes air better than weather as an abstract concept. Finnish naturally says ilma on lämmin.

Because lämmin is a predicate adjective linked to the singular subject ilma.

With a singular subject, the predicate adjective is often in the nominative:

• ilma on lämmin
• talo on iso
• mehu on kylmä

So lämmin agrees in a basic predicate sense with ilma.

You may also see partitive forms like on lämmintä, but that is a different structure, often used in more impersonal expressions such as:

• Ulkona on lämmintä. = It is warm outside.

That version does not have the same clear subject structure as ilma on lämmin.

Yes. Finnish word order is fairly flexible, although the neutral order here is very natural.

You can also say:

• Kun ilma on lämmin, laitan kylmää mehua lasiin.

This puts the when-clause first and sounds just as natural.

Changing word order can affect emphasis more than basic meaning. Finnish uses case endings to show grammatical relationships, so word order is freer than in English.

It can mean present or habitual here, depending on context.

• Laitan kylmää mehua lasiin, kun ilma on lämmin. can mean something like
  I put/pour cold juice into a glass when the weather is warm
  = a general habit

Finnish has no separate future tense, so the present can also refer to the future when the context makes that clear.

For example:

• Huomenna laitan mehua lasiin. = Tomorrow I’ll put/pour juice into a glass.

So Finnish present tense is broader than English present tense.

Because Finnish normally uses a comma to separate a main clause from a subordinate clause.

Here:

• main clause: Laitan kylmää mehua lasiin
• subordinate clause: kun ilma on lämmin

So the comma is standard Finnish punctuation.

If the order is reversed, the comma still appears:

• Kun ilma on lämmin, laitan kylmää mehua lasiin.

No. In this sentence, kylmää mehua is the thing being put or poured, so that is the object-like element in partitive form.

lasiin is not the object; it expresses the destination:

• into a glass

So the sentence has:

• what? → kylmää mehua
• where to? → lasiin

That distinction is very important in Finnish, because cases often show these roles clearly.