Laitan laukun maahan, koska se on raskas.

Because the verb ending already shows the subject.

• laitan = I put / I place
• the ending -n marks first person singular

So Minä laitan laukun maahan is also possible, but minä is usually left out unless you want emphasis or contrast.

Because laukku is the object, and in this sentence it is a total object: the action is seen as completed and directed at the whole bag.

So:

• laukku = bag
• laukun = the bag as a singular total object

In many Finnish sentences like this, the singular total object looks like the genitive form.

So Laitan laukun maahan means you are putting the whole bag down.

A useful contrast:

• Laitan laukun maahan = I put the bag on the ground
• En laita laukkua maahan = I am not putting the bag on the ground

In negative sentences, the object usually becomes partitive, so laukkua appears instead.

Maahan is the illative form of maa.

The illative case often means into, onto, or more generally to a place as a destination.

Here the idea is movement toward the ground, so Finnish uses a case ending instead of a preposition like to or onto.

• maa = ground / land / earth
• maahan = into the ground / onto the ground / to the ground

In natural English, this sentence is usually translated with on the ground or down on the ground, even though Finnish is using a directional case.

Yes, here maa means ground.

Maa is a broad word and can mean:

• earth
• land
• country
• ground

The context tells you which meaning is intended.

In Laitan laukun maahan, the verb laitan and the idea of physically placing a bag make ground the natural meaning.

Here se refers to the bag.

So the logic is:

• Laitan laukun maahan = I put the bag on the ground
• koska se on raskas = because it is heavy

So se = it, meaning the bag.

Finnish does not have grammatical gender like he/she in the same way English does. Also, in everyday spoken Finnish, se is often used for people too, but in this sentence it clearly refers to the bag.

Because raskas is a predicate adjective after the verb on.

In se on raskas, the adjective describes the subject se. In this kind of sentence, the adjective is normally in its basic dictionary form when the subject is singular.

So:

• se = it
• on = is
• raskas = heavy

The adjective is not matching laukun here, because laukun is in the first clause and functions as an object there. In the second clause, raskas describes se.

Because koska introduces a subordinate clause.

Finnish standard writing normally separates the main clause and the subordinate clause with a comma.

So:

• Laitan laukun maahan = main clause
• koska se on raskas = subordinate clause

That is why the comma appears before koska.

Not completely, but the given order is the most neutral one.

Finnish word order is more flexible than English word order because case endings show what words are doing in the sentence. Still, some orders sound more neutral than others.

• Laitan laukun maahan, koska se on raskas. = neutral, natural
• Laukun laitan maahan... = emphasizes the bag
• Maahan laitan laukun... = emphasizes onto the ground

So the original sentence is the ordinary, unmarked way to say it.

Yes, if you specifically mean onto the floor.

• maahan = onto the ground / down on the ground / sometimes onto the floor in context
• lattialle = onto the floor

So:

• Laitan laukun maahan can sound a bit more general: I put the bag down on the ground.
• Laitan laukun lattialle is more specific: I put the bag on the floor.

If you are indoors and clearly mean the floor, lattialle may be the more precise choice.

Here laittaa means to put, to place, or to set.

It is a very common Finnish verb with a broad range of uses. Depending on context, it can mean things like:

• put
• place
• set
• install
• arrange

In this sentence, it has the simple physical meaning put/place:

• laitan laukun maahan = I put the bag on the ground

So this is a very natural everyday use of laittaa.