A nadrágom tiszta, de a zoknim még mindig a széken van.

Because Hungarian can use the definite article together with a possessed noun.

So:

• nadrágom = my pants / my trousers
• a nadrágom = my pants in a definite, specific sense

This is normal Hungarian. Unlike English, the-type article and possessive marking are not mutually exclusive.

They are 1st person singular possessive endings, meaning my.

• nadrág
  • -om → nadrágom = my pants / my trousers
• zokni
  • -m → zoknim = my sock

Hungarian usually shows possession with a suffix on the noun rather than with a separate word like my.

Because zokni ends in a vowel, and with many vowel-final nouns the possessive ending is simply -m.

So:

• zokni → zoknim
• compare autó → autóm

You do not add an extra full -om after the final vowel here.

In Hungarian, the verb to be is usually left out in the present tense, 3rd person when the predicate is an adjective or a noun.

So:

• A nadrágom tiszta. = My pants are clean.

Literally, it looks more like:

• My pants clean.

This is one of the most common differences from English.

Because with location or existence, Hungarian normally does use van in the present tense.

• A zoknim a széken van. = My sock is on the chair.

So there is a difference between:

• A nadrágom tiszta. = adjective predicate → no van
• A zoknim a széken van. = location predicate → van is used

még mindig means still.

In this sentence:

• a zoknim még mindig a széken van = my sock is still on the chair

This is a very common Hungarian expression.

A useful rough guide is:

• még can mean still / yet / even
• mindig usually means always
• még mindig together means still, even now

So the combination is the natural way to say that something has continued up to now.

szék means chair, and -en is the case ending meaning on.

So:

• szék = chair
• széken = on the chair

This case is called the superessive.

The article a means the speaker has a specific chair in mind:

• a széken = on the chair

Because Hungarian treats nadrág as a singular noun for the whole garment.

English says:

• pants
• trousers

But Hungarian normally says:

• nadrág

So A nadrágom tiszta is literally singular in Hungarian, even though English uses a plural word.

Yes, zoknim is singular:

• zoknim = my sock

If you want my socks, you need a plural possessed form, for example:

• zoknijaim = my socks

So this sentence is talking about one sock, not a pair of socks.

Yes, Hungarian word order is flexible, but different orders change the emphasis.

This sentence has a fairly neutral structure:

• A zoknim = topic, what we are talking about
• még mindig a széken van = what is being said about it

So the sense is:

• As for my sock, it is still on the chair.

You could also say:

• Még mindig a széken van a zoknim.

That would sound more emphatic, highlighting the fact that the sock is still there.

Because in this sentence tiszta is a predicate adjective, and with a singular subject it stays in its basic singular form.

So:

• A nadrágom tiszta. = My pants are clean.

It does not take a special ending for my. The possession is already shown on nadrágom, not on the adjective.

de means but.

It connects the two contrasting ideas:

• A nadrágom tiszta = My pants are clean
• de = but
• a zoknim még mindig a széken van = my sock is still on the chair

So the whole sentence contrasts one good fact with one annoying or unfinished fact.