Sada lepení, co jsi koupil, má dvě záplaty.

Yes, co very often means what, but in everyday Czech it is also commonly used as a relative word meaning that / which.

So:

• co jsi koupil = that you bought

In more formal or careful Czech, many speakers would prefer:

• kterou jsi koupil

That is because the relative word refers back to sada and is the object of koupit inside the clause.

So you can compare:

• Sada lepení, co jsi koupil, ... = more colloquial
• Sada lepení, kterou jsi koupil, ... = more formal/standard

Both are understandable.

Because inside the relative clause, the kit is the object of the verb koupil.

You bought the kit:

• koupil jsi sadu = you bought the kit

Since sada is feminine singular, the relative pronoun has to match it and also take the correct case:

• nominative: která
• accusative: kterou

Here we need the accusative, so:

• kterou jsi koupil = which/that you bought

Because it is a subordinate clause, and Czech normally separates subordinate clauses with commas.

So the structure is:

• Sada lepení = main subject
• co jsi koupil = inserted relative clause
• má dvě záplaty = main clause continues

That is why you get:

• Sada lepení, co jsi koupil, má dvě záplaty.

This is different from English, where commas depend more on whether the clause is restrictive or non-restrictive. In Czech, subordinate clauses are generally marked off with commas much more consistently.

Because koupil agrees with the hidden subject ty in jsi koupil, not with sada.

• jsi koupil = you bought
• jsi already tells you the subject is you singular

So:

• if the speaker is male: jsi koupil
• if the speaker is female: jsi koupila

It does not agree with sada, because sada is not the subject of that clause. It is the thing being bought.

Because the subject of the main clause is sada, which is singular.

• sada = set / kit
• má = has

Even though the kit contains two patches, the subject is still just one kit, so the verb stays singular:

• Sada ... má dvě záplaty.

Sada is in the nominative singular because it is the subject of the sentence.

• sada = nominative singular

Lepení is a verbal noun related to lepit (to glue / to patch). In this phrase it depends on sada, something like a kit for gluing/patching.

Formally, you can think of it as the noun attached to sada. But because lepení has the same form in several cases, the ending does not visibly change here.

The important practical point for a learner is:

• sada = the head noun
• lepení = specifies what kind of set/kit it is

It is understandable, but many native speakers would probably say something more natural, for example:

• sada na lepení
• lepicí sada
• in some contexts, opravná sada

So if you mean something like a patch kit or repair kit, sada na lepení is probably the safest everyday choice.

That means a very natural version of the whole sentence would be:

• Sada na lepení, kterou jsi koupil, má dvě záplaty.

The original sentence is still useful for learning, especially for the grammar, but the noun phrase itself sounds a bit less idiomatic.

Because záplata is a feminine noun.

With the number 2, Czech uses:

• dva for masculine nouns
• dvě for feminine and neuter nouns

So:

• dva hrady = two castles
• dvě ženy = two women
• dvě města = two towns
• dvě záplaty = two patches

Since záplata is feminine, dvě is correct.

It is the direct object of má, so the whole phrase is functioning as accusative.

After 2, 3, 4, Czech uses the plural noun form, so:

• dvě záplaty = two patches

For this noun, the form záplaty is the expected plural form here.

So in practice, you can learn it as:

• má dvě záplaty = has two patches

The key thing to remember is that after dvě, Czech uses a plural noun, not a singular one.

Yes. Czech word order is flexible, though not completely free. This sentence is fine, but other orders are possible depending on emphasis.

For example:

• Sada lepení, co jsi koupil, má dvě záplaty.
• Ta sada lepení, co jsi koupil, má dvě záplaty.

If you make the noun phrase more natural, you might also hear:

• Sada na lepení, kterou jsi koupil, má dvě záplaty.

The relative clause usually stays near the noun it describes, so co jsi koupil or kterou jsi koupil naturally follows sada.