Attól függően, hogy melyik szemüvegkeret kényelmes, azt próbálom fel.

Attól függően is a set expression meaning depending on that / depending on...

It is built from:

• az = that
• attól = from that / on that, with the -tól/-től ending
• függően = depending, from the verb függ (to depend)

So even though attól literally looks like from that, the whole expression should be learned together as attól függően = depending on.

It is very often followed by a clause with hogy:

• Attól függően, hogy... = Depending on whether/how/which...

Hungarian usually uses hogy to introduce this kind of subordinate clause.

So:

• Attól függően, hogy... = Depending on...

In English, we often go straight to which:

• depending on which frame is comfortable

Hungarian typically keeps hogy before the embedded question:

• attól függően, hogy melyik...

This is very normal and idiomatic.

Melyik means which one and is used when choosing from a limited set of options.

That fits this sentence well, because the speaker is choosing among several eyeglass frames:

• melyik szemüvegkeret = which frame

A native English speaker may wonder why it is not mi. That is because mi means what, not which one out of several.

So:

• mi? = what?
• melyik? = which one?

Sometimes yes, but it changes the structure slightly.

• melyik is the natural choice in an embedded selection question:
  hogy melyik szemüvegkeret kényelmes
• amelyik often means the one that / whichever

So a related sentence would be:

• Amelyik szemüvegkeret kényelmes, azt próbálom fel.
  = Whichever frame is comfortable, I try that one on.

In your original sentence, melyik fits well because the clause is framed as which one is comfortable.

In Hungarian, the verb to be is usually omitted in the third person singular present tense when linking a subject to a noun or adjective.

So:

• A keret kényelmes. = The frame is comfortable.
• literally: The frame comfortable.

That is standard Hungarian grammar.

So in:

• melyik szemüvegkeret kényelmes the meaning is which eyeglass frame is comfortable, even though there is no visible word for is.

Azt means that or that one, and here it refers back to the frame identified in the first clause.

The logic is:

• which frame is comfortable
• that one I try on

Hungarian often uses this kind of resumptive pronoun:

• melyik... azt...
• which one... that one...

So azt is not unnecessary. It helps clearly point back to the chosen frame.

Because Hungarian verbs have indefinite and definite conjugation.

Here the object is definite:

• azt = that one

Since the speaker is trying on a specific, definite object, the verb must be in the definite conjugation:

• próbálom = I try it / I am trying it

Compare:

• Próbálok egy keretet. — not natural in this exact meaning, but structurally indefinite
• Azt a keretet próbálom. = definite object, so próbálom

So azt triggers próbálom.

The basic verb is felpróbál = to try on.

In a neutral sentence, the prefix usually stays attached:

• Felpróbálom. = I try it on.

But when another element is focused or highlighted before the verb, the prefix often moves after the verb:

• Azt próbálom fel.

This is a very common Hungarian pattern with verbal prefixes.

So:

• felpróbálom = neutral
• azt próbálom fel = the object azt gets emphasis: that one is what I try on

They are close in basic meaning, but they do not sound equally natural in the same context.

• azt próbálom fel is the more natural version here
• felpróbálom azt is possible, but sounds more marked or less idiomatic in many contexts

Why? Because Hungarian word order is strongly connected to focus and information structure. In this sentence, azt is the important element: that one is the one being tried on.

So the split-prefix version is the better fit here:

• azt próbálom fel

Because melyik is asking about one item out of several.

So:

• melyik szemüvegkeret = which eyeglass frame

Even if there are many frames available in the shop, the speaker is choosing one at a time, so the singular is natural.

If you wanted to talk about multiple frames as plural items, you could say:

• melyik szemüvegkeretek...

But in this sentence, singular makes perfect sense.

The sentence is organized like this:

1. Attól függően = sets up the idea of dependence
2. hogy melyik szemüvegkeret kényelmes = gives the condition or basis for the choice
3. azt próbálom fel = tells what the speaker does with the chosen item

The most important word-order point is in the main clause:

• azt próbálom fel

This puts attention on azt: that one is the frame the speaker tries on.

So the word order is not random. It helps show what is being selected and emphasized.