Laitan avaimen laukkuun, jotta en unohda sitä.

Finnish often leaves out subject pronouns when the verb already shows the person clearly.

• laitan = I put / I’ll put
• en unohda = I do not forget / I won’t forget

So minä is understood and does not need to be said. You could add it for emphasis, but the neutral version usually leaves it out:

• Minä laitan avaimen laukkuun = more emphatic
• Laitan avaimen laukkuun = normal

Finnish very often uses the present tense for future meaning when the context is clear.

So laitan can mean:

• I put
• I’m putting
• I’ll put

In this sentence, it naturally refers to a near-future action or intention. Finnish does not usually need a separate future tense.

Because avaimen is the object form used here.

The basic form is avain = key.
In this sentence, the key is the whole object of the action laitan. In an affirmative sentence like this, a singular total object usually appears in a form that looks like the genitive:

• avain = basic form
• avaimen = total object here

So:

• Laitan avaimen laukkuun = I put the key into the bag

If you used avainta, that would be partitive and would suggest a different kind of object meaning, not the natural one here.

Laukkuun is the illative form of laukku, and the illative often means into something.

• laukku = bag
• laukkuun = into the bag

So this part tells you the direction of movement: the key is being put into the bag.

Useful comparison:

• laukussa = in the bag
• laukkuun = into the bag
• laukusta = out of the bag

Because jotta introduces a subordinate clause, and in standard Finnish, subordinate clauses are normally separated with a comma.

So the structure is:

• main clause: Laitan avaimen laukkuun
• subordinate clause: jotta en unohda sitä

Finnish uses this comma more regularly than English does in some similar sentences.

Jotta means so that or in order that. It introduces a purpose clause.

So the second clause explains the purpose of the first action:

• Laitan avaimen laukkuun
• jotta en unohda sitä

In other words: I do the first thing for the purpose of the second thing not happening.

In everyday Finnish, people may also use other constructions, for example etten in similar sentences:

• Laitan avaimen laukkuun, etten unohda sitä.

That is also natural. Jotta sounds a bit more explicit and often a bit more written or careful.

Finnish makes negation with a separate negative verb plus the main verb in a special form.

Here:

• en = negative verb for I
• unohda = the main verb in its negative form

Compare:

• unohdan = I forget
• en unohda = I do not forget / I won’t forget

This is a very important Finnish pattern:

• en
• et
• ei
• emme
• ette
• eivät

So the person marking moves onto the negative verb.

Because the clause is negative, and in Finnish the object of a negative verb is normally in the partitive.

Compare:

• Unohdan sen = I forget it
• En unohda sitä = I do not forget it

So in your sentence, sitä is required because of en.

This is one of the most useful rules to remember: negative clause -> object usually in the partitive

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

So Finnish simply says:

• avain / avaimen
• laukku / laukkuun

and the exact English choice depends on context:

• a key
• the key
• a bag
• the bag

In this sentence, English often uses the key and the bag because the context usually makes them specific, but Finnish does not mark that with articles.

The sentence as given has a very normal word order, but Finnish word order is somewhat flexible.

This version is neutral:

• Laitan avaimen laukkuun, jotta en unohda sitä.

You could also move the purpose clause first:

• Jotta en unohda sitä, laitan avaimen laukkuun.

That changes the focus slightly, but the meaning stays the same.

Inside the negative clause, though, en unohda stays together as the normal negative verb pattern. So Finnish word order is flexible, but not random.