Todavía me cuesta distinguir algunas preposiciones cuando hablo deprisa.

Here todavía means still.

So Todavía me cuesta... means I still find it hard...

In other contexts, todavía can sometimes mean yet, but in this sentence still is the natural translation.

Examples:

• Todavía estudio español. = I still study Spanish.
• Todavía no lo entiendo. = I still don’t understand it / I don’t understand it yet.

Because Spanish uses costar in a structure a bit like gustar.

Literally, me cuesta means it is difficult for me or it costs me effort.

The idea is not I cost, but rather:

• me = to me / for me
• cuesta = is difficult / costs effort

So:

• Me cuesta distinguir... = It’s hard for me to distinguish...
• more naturally in English: I find it hard to distinguish...

The subject is the whole infinitive phrase:

distinguir algunas preposiciones cuando hablo deprisa

That entire action is what costs effort.

So the sentence works like this:

• Todavía me cuesta = I still find it hard
• distinguir algunas preposiciones cuando hablo deprisa = to distinguish some prepositions when I speak quickly

Because an infinitive phrase is treated as singular, Spanish uses cuesta (singular), not cuestan.

Costar + infinitive is a very common way to say to find something hard / to struggle to do something.

Pattern:

• me cuesta + infinitive = I find it hard to...
• te cuesta + infinitive = you find it hard to...
• le cuesta + infinitive = he/she finds it hard to...

Examples:

• Me cuesta entender el acento. = I find it hard to understand the accent.
• Nos cuesta recordar las reglas. = We find it hard to remember the rules.

It often sounds more natural in Spanish than directly saying es difícil para mí.

Yes, you could.

Es difícil para mí distinguir algunas preposiciones... is grammatically correct and understandable.

But me cuesta distinguir... is more natural and idiomatic in everyday Spanish for personal difficulty. It often suggests ongoing effort or struggle, not just abstract difficulty.

So:

• Es difícil para mí... = more neutral / descriptive
• Me cuesta... = more personal / experiential

Distinguir means to distinguish, to tell apart, or to notice the difference between things.

In this sentence, it means the speaker has trouble telling certain prepositions apart when speaking fast.

It is a very natural verb here.

Close alternatives include:

• diferenciar = to differentiate
• separar is not right here
• reconocer = to recognize, which is a different idea

If you want to be more explicit, you could say:

• Me cuesta distinguir unas preposiciones de otras. = I find it hard to tell some prepositions apart from others.

Because algunas preposiciones means some prepositions, not all of them.

The sentence is saying the speaker has trouble with certain prepositions, an indefinite subset.

Compare:

• algunas preposiciones = some prepositions
• las preposiciones = the prepositions / prepositions in general

So algunas makes the sentence more specific and more natural if the speaker only struggles with certain ones.

Because it describes a general, repeated situation:

when I speak quickly

The present tense in Spanish is often used for habits or recurring situations, just like in English.

So cuando hablo deprisa means:

• when(ever) I speak quickly
• when I’m speaking fast, in a general sense

It does not refer only to one specific moment.

All three can work, but they are not exactly the same in feel.

• deprisa = quickly / fast
• rápido = often used adverbially in speech, especially in informal language
• rápidamente = quickly, but more formal or more explicitly adverbial

In Spain, hablar deprisa is very common and natural.

So:

• cuando hablo deprisa = very natural
• cuando hablo rápido = also common in everyday speech
• cuando hablo rápidamente = correct, but less conversational here

Yes. In this sentence, deprisa is written as one word, and that is the standard spelling.

It means quickly / in a hurry / fast.

Examples:

• Voy deprisa. = I’m going quickly / I’m in a hurry.
• No hables tan deprisa. = Don’t speak so fast.

Yes. In this sentence, todavía and aún are interchangeable.

So you could also say:

Aún me cuesta distinguir algunas preposiciones cuando hablo deprisa.

That means the same thing: I still find it hard to distinguish some prepositions when I speak quickly.

In everyday speech, todavía is often more common, but both are correct.

A useful point here is that the u in gui is normally silent.

So distinguir is pronounced roughly like:

dees-teen-GEER

Important points:

• the g in gui is a hard g sound, like g in get
• the u is not pronounced
• the stress falls on the last syllable: -guir

Also in this sentence:

• h in hablo is silent
• deprisa is pronounced roughly deh-PREE-sah

Yes, absolutely.

Cuando hablo rápido is very common and natural in spoken Spanish.

The version with deprisa may sound a bit more idiomatic in Spain, but both are normal.

So both of these work:

• cuando hablo deprisa
• cuando hablo rápido

If you are learning Spanish as used in Spain, deprisa is especially worth knowing.

Yes, a little.

The original sentence:

Todavía me cuesta distinguir algunas preposiciones cuando hablo deprisa.

is very natural.

You could also say:

• Me cuesta todavía distinguir algunas preposiciones cuando hablo deprisa.
• Me cuesta distinguir algunas preposiciones cuando hablo deprisa, todavía.

But starting with Todavía is the smoothest and most natural way to emphasize still.

So the original order is probably the best model to learn.

Yes, there is a small difference.

• cuando hablo deprisa = when I speak quickly
  This sounds like a usual or repeated situation.
• si hablo deprisa = if I speak quickly
  This sounds more conditional.

In your sentence, cuando is the better choice because the speaker means this happens in those situations where they speak fast, not that it is just a hypothetical possibility.

Not in exactly the same way.

Distinguir can sometimes be followed by entre or by de, but the structure changes slightly.

Natural options are:

• Me cuesta distinguir algunas preposiciones.
• Me cuesta distinguir unas preposiciones de otras.

The second one is especially clear if you want to emphasize telling them apart from one another.

Distinguir entre algunas preposiciones sounds less natural here than the original sentence.