Što bolje poravnaš dasku, to je komoda stabilnija.

This is a very common correlative comparative pattern in Serbian.

Što X, to Y means:

• the more X, the more Y
• the better X, the more Y
• the less X, the less Y

So here:

Što bolje poravnaš dasku, to je komoda stabilnija.
= The better you level the board, the more stable the chest of drawers is.

In this pattern, što and to do not literally mean what and that. Together they create the the ... the ... relationship.

Because bolje is an adverb, and it modifies the verb poravnaš.

The sentence is talking about how you level the board:

• dobro = well
• bolje = better

So:

• bolje poravnaš = you level it better

By contrast, bolji is an adjective, used to describe a noun:

• bolji sto = a better table

Here we are not describing daska as better; we are describing the action of leveling.

Because stabilnija is the comparative form of the adjective stabilna and it agrees with komoda.

• komoda is feminine singular
• so the adjective must also be feminine singular

Forms:

• stabilan = stable (masculine)
• stabilna = stable (feminine)
• stabilniji / stabilnija / stabilnije = more stable

So:

• komoda je stabilnija = the chest of drawers is more stable

Dasku is the accusative singular of daska.

That is because it is the direct object of the verb poravnaš:

• daska = board, plank
• poravnati dasku = to level the board

So:

• nominative: daska
• accusative: dasku

This is normal for a feminine noun ending in -a.

Poravnaš is:

• 2nd person singular
• present tense
• from the verb poravnati

So literally it means you level / you make level.

The subject ti is not stated, because Serbian often leaves subject pronouns out when the verb ending already shows who the subject is.

So:

• poravnaš = you level

In this kind of sentence, the present tense does not have to mean an action happening right now. It often has a general meaning:

• the better you level the board...

Serbian often uses a perfective present in clauses that express a general condition, result, or repeated relationship, especially in patterns like this.

Poravnati is a perfective verb, meaning a completed leveling action. In this sentence, the idea is not you are leveling it right now, but rather:

• whenever / if you level it better
• the better you manage to level it

So the perfective verb fits the idea of reaching a result.

An imperfective version could appear in other contexts, but poravnaš sounds natural here because the sentence focuses on the achieved outcome.

Because komoda stabilnija needs the verb to be in Serbian:

• komoda je stabilnija = the chest of drawers is more stable

Unlike some Slavic languages, standard Serbian normally does use the present tense of biti in this kind of sentence.

So je is not optional in normal standard usage here.

Because je is a clitic. Serbian clitics usually go in the second position of their clause.

In the second clause:

• to comes first
• so je comes right after it

That gives:

• to je komoda stabilnija

This word order is very typical in Serbian.

So the order is not random; it follows the normal placement rule for unstressed words like je.

It is the same form, što, but not the same function.

Što can mean different things depending on context, for example:

• what
• that/which
• part of a fixed expression like što ..., to ...

In this sentence, što is not asking a question and does not mean what. It is part of the comparative pattern što ..., to ....

So you should understand it here as part of the ... the ..., not as an interrogative word.

The most standard pattern is exactly what you see here:

Što + comparative expression ..., to + main clause ...

So this sentence is a very natural model.

In speech and other sentences, Serbian can sometimes be a bit flexible, and to may be omitted in some contexts. But for learners, it is best to treat što ..., to ... as the safe, standard structure.

So if you want to build similar sentences, copy this pattern:

• Što više učiš, to više znaš.
• Što brže radiš, to ranije završiš.

That will sound natural and correct.