Laitoin kirjekuoren päälle kaksi postimerkkiä, jotta kirje menee varmasti perille.

Laitoin is the 1st person singular past tense of laittaa.

• laittaa = the dictionary form
• laitoin = I put / I placed

So the ending -in tells you the subject is I and the action happened in the past.

Finnish often leaves out subject pronouns when they are clear from the verb ending.

So:

• laitoin already means I put
• you do not need minä unless you want extra emphasis or contrast

For example:

• Laitoin kirjekuoren päälle kaksi postimerkkiä. = neutral
• Minä laitoin kirjekuoren päälle kaksi postimerkkiä. = emphasizes I

This is very common in Finnish.

Because päälle is a postposition, and postpositions usually require the noun before them to be in the genitive.

So:

• kirjekuori = envelope
• kirjekuoren = of the envelope / the envelope's form, i.e. genitive singular

In this sentence:

• kirjekuoren päälle = onto/on top of the envelope

This is a normal Finnish pattern:

• pöydän päälle = onto the table
• tuolin alle = under the chair
• talon eteen = in front of the house

This is a common noun type in Finnish.

• basic form: kirjekuori
• genitive: kirjekuoren

The final -i often changes in oblique forms, so the stem becomes kirjekuore- and then gets the genitive ending -n.

So:

• kirjekuori
• stem: kirjekuore-
• genitive: kirjekuoren

Learners often just have to get used to this noun pattern by seeing many examples.

Because päälle expresses movement onto a surface, while päällä expresses location on a surface.

Here the action is putting the stamps onto the envelope, so Finnish uses the directional form:

• päälle = onto, on top of
• päällä = on, on top of

Compare:

• Laitoin postimerkin kirjekuoren päälle. = I put the stamp onto the envelope.
• Postimerkki on kirjekuoren päällä. = The stamp is on the envelope.

This movement-vs-location distinction is very important in Finnish.

Because a stamp goes on the surface of an envelope, not into it.

• kirjekuoren päälle = onto the envelope / on top of the envelope
• kirjekuoreen = into the envelope

So the Finnish matches the physical situation very closely.

After numbers greater than one, Finnish usually puts the counted noun in the singular partitive.

So:

• kaksi postimerkkiä = two stamps
• not kaksi postimerkkejä

This is one of the most important number rules in Finnish:

• yksi postimerkki = one stamp
• kaksi postimerkkiä = two stamps
• kolme postimerkkiä = three stamps

Even though the meaning is plural in English, Finnish uses singular partitive after the numeral.

Because that is the normal pattern after kaksi and other numerals above one.

In numeral phrases:

• the number word carries the idea of plurality
• the noun that follows is usually singular partitive

So:

• kaksi postimerkkiä
• viisi kirjettä
• kolme taloa

This is not because the stamps are somehow incomplete or indefinite in the usual partitive sense. It is mainly because numerals require this structure.

Jotta introduces a purpose clause.

It means something like:

• so that
• in order that

So the sentence structure is:

• main action: I put two stamps on the envelope
• purpose: so that the letter will definitely arrive

This is a very common way to express purpose in Finnish.

This is a good question, because learners often see both.

In traditional grammar and in more careful/formal style, a purpose clause with jotta often uses the conditional:

• jotta kirje menisi varmasti perille

But in everyday modern Finnish, the indicative is also common:

• jotta kirje menee varmasti perille

So in this sentence, menee is understandable and natural in modern usage.
If you used menisi, it could sound a bit more explicitly like in order that the letter would arrive.

For a learner, the safest takeaway is:

• jotta + conditional = very common in textbooks and formal explanations
• jotta + indicative = also common in real usage

Perille is part of the expression mennä perille, which means to arrive, to get through, or to reach its destination.

So:

• mennä alone = to go
• mennä perille = to get there / to arrive successfully

With letters, messages, calls, and similar things, perille often adds the idea of reaching the intended destination.

Examples:

• Kirje meni perille. = The letter arrived.
• Viesti ei mennyt perille. = The message didn’t get through.

So perille is important here; it is not just extra decoration.

Varmasti is an adverb meaning certainly / definitely / surely.

Its position is quite natural here because it modifies the whole idea of getting to its destination:

• kirje menee varmasti perille

This placement sounds smooth and idiomatic. Finnish word order is fairly flexible, but different positions can slightly change emphasis.

For example:

• jotta kirje menee varmasti perille = neutral, natural
• jotta kirje varmasti menee perille = extra emphasis on certainty

Both are possible, but the sentence you have is very normal.

Finnish does not have articles like English a/an and the.

So:

• kirje can mean a letter or the letter
• kirjekuori can mean an envelope or the envelope

The exact meaning comes from context.

This is why Finnish learners from English often feel that something is missing, but for Finnish, nothing is missing at all.

Yes, normally it is.

In Finnish, subordinate clauses are typically separated from the main clause with a comma, and jotta introduces a subordinate clause.

So the comma in:

• Laitoin kirjekuoren päälle kaksi postimerkkiä, jotta kirje menee varmasti perille.

is standard Finnish punctuation.

Yes, absolutely. It is very natural.

Finnish often uses very concrete spatial expressions, and a stamp is literally placed onto the surface of the envelope, so kirjekuoren päälle fits well.

Depending on style and context, you might also hear other ways of expressing similar ideas, but this sentence is perfectly normal and idiomatic.

Yes, Finnish word order is flexible, though not random.

For example, you could rearrange parts of the sentence for emphasis, but the original version is very neutral and natural:

• Laitoin kirjekuoren päälle kaksi postimerkkiä, jotta kirje menee varmasti perille.

If you move things around, you often change what is being emphasized rather than the basic meaning.

So for learners, this sentence is a good model of ordinary, straightforward word order.