Learn Hardcore Spanish (Spain): No entiendo su expresión cuando hablo tan rápido en español. - I don’t understand her expression when I speak so fast in Spanish.

You cannot put no at the end of the sentence, as in English “I understand not.” That sounds archaic or poetic in English and is simply wrong in normal Spanish.

So No entiendo su expresión… is the standard, correct word order.

Why is there no yo? Could I say Yo no entiendo su expresión…?

Spanish usually omits subject pronouns because the verb ending already shows who the subject is:

entiendo = I understand
entiendes = you (tú) understand
entiende = he/she/you (usted) understands

So (Yo) no entiendo su expresión… both mean “I don’t understand their/your expression…”.

You only need yo if you want to emphasize I, for example:

Yo no entiendo su expresión, pero ellos sí.
I don’t understand their expression, but they do.

In your sentence, leaving out yo is the most natural, neutral option.

What exactly does su mean here? Is it his, her, their or your? Why not tu?

Su is ambiguous by itself; it can mean:

his expression
her expression
their expression
your expression (formal usted / ustedes)

Context normally makes it clear.

Su vs tu is mainly about formality and number:

tu expresión – your expression (talking to one person, informal: tú)
su expresión – your expression (talking to one person formally – usted, or to more than one person: ustedes)

Also, su expresión can refer to someone being talked about (his/her/their).

So:

Talking informally to a friend:
- No entiendo tu expresión…
Talking formally to a stranger/boss/customer:
- No entiendo su expresión…
Talking about another person:
- No entiendo su expresión cuando él habla tan rápido… (his expression)

Does su expresión really sound like their / his / her facial expression? Wouldn’t Spanish say something with cara?

Yes, su expresión can naturally mean their facial expression, his expression, her expression, etc. However, Spanish often makes it more explicit:

No entiendo la expresión de su cara… – I don’t understand the expression on their/your face.
No entiendo la cara que pones cuando… – I don’t understand the face you make when…

All of these are idiomatic.

su expresión is a bit more general (could be facial, emotional, the way they express themselves, etc.).
la expresión de su cara or la cara que pones clearly refers to the face.

In everyday conversation, la cara que pones is very common and very natural in Spain.

Why is expresión singular and not plural (expresiones)? In English I might say their expressions.

Spanish often uses the singular where English might use the plural for a general type of thing.

Here su expresión refers to the way their face looks in that situation (one “type” of expression), so singular is more natural:

No entiendo su expresión cuando hablo tan rápido…
= I don’t understand the way their face looks / the expression they get when I speak so fast.

If you said sus expresiones, that would suggest several different expressions (many faces they make), which is a different nuance.

Why is it cuando hablo and not cuando estoy hablando? Is that possible too?

Cuando hablo tan rápido en español uses the present simple to talk about a repeated / habitual situation:

Whenever I speak that fast in Spanish, I don’t understand their expression.

You can say cuando estoy hablando tan rápido en español, but it shifts the focus slightly toward an action in progress at that moment:

No entiendo su expresión cuando estoy hablando tan rápido en español.
Sounds a bit more like: I don’t understand their expression at the time I’m in the middle of speaking so fast.

For a general, repeated situation, cuando hablo is more common and natural.

Could I replace cuando hablo tan rápido with al hablar tan rápido?

Yes, you can:

No entiendo su expresión cuando hablo tan rápido en español.
No entiendo su expresión al hablar tan rápido en español.

Both are correct, but there’s a nuance:

cuando hablo – “when I speak”; a bit more direct and conversational.
al hablar – literally “on (upon) speaking”; slightly more formal/literary, often used in more careful or written Spanish.

In everyday speech, cuando hablo is the more neutral, natural choice.

What’s the difference between tan rápido, muy rápido, and demasiado rápido here?

All three are possible, but they’re not the same:

tan rápido – so fast
- Often implies a comparison or result (even if it’s only implied):
  - Hablo tan rápido que no entiendes. – I speak so fast that you don’t understand.
muy rápido – very fast
- Just emphasizes a high degree, without necessarily implying a consequence.
demasiado rápido – too fast
- Explicitly says it’s excessive, more than is good/appropriate.

In your sentence:

…cuando hablo tan rápido en español. – so fast (that it causes this reaction).
…cuando hablo muy rápido en español. – very fast.
…cuando hablo demasiado rápido en español. – too fast (overly fast).

In Spain you’ll also hear deprisa:

cuando hablo tan deprisa en español. – very idiomatic there.

Why is it en español and not en el español? And what about castellano?

With languages, Spanish normally uses the preposition en without an article:

en español – in Spanish
en inglés – in English
en francés – in French

En el español exists, but it has a different meaning: it refers to the Spanish language as an object of study, like “in the Spanish language”:

Hay muchos dialectos en el español. – There are many dialects in the Spanish language.

Here you’re just saying you speak in Spanish, so it’s en español.

As for español vs castellano:

In many parts of Spain, español and castellano are both used for “Spanish”.
Hablar en castellano is very common in Spain; it would be fine to say:
- …cuando hablo tan rápido en castellano.

Meaning-wise, in this everyday context, en español and en castellano are equivalent.

Can I change the word order of tan rápido and en español? For example: cuando hablo en español tan rápido?

Yes, Spanish word order is somewhat flexible, and both are possible:

cuando hablo tan rápido en español
cuando hablo en español tan rápido

The first one (tan rápido directly after hablo) sounds a bit more natural and fluent in everyday speech, because tan rápido is closely tied to hablo (“speak so fast”).

The second is not wrong; it just slightly changes the internal rhythm of the sentence. In practice, you’ll most often hear:

cuando hablo tan rápido en español.

Why No entiendo su expresión and not No comprendo su expresión or No puedo entender su expresión?

All are grammatically correct, but they have different nuances:

No entiendo su expresión…
- Most natural, everyday choice.
- entender is used very broadly for understanding language, people, situations, etc.
No comprendo su expresión…
- Sounds more formal, logical, or a bit “bookish” in many contexts.
- In everyday speech, comprender is less frequent than entender, especially in Spain.
No puedo entender su expresión…
- Emphasizes inability: “I can’t manage to understand their expression.”
- Suggests you try or would like to understand but are unable to.

In your specific sentence, No entiendo su expresión is what a Spaniard would most naturally say in casual conversation.

Could a Spaniard say No le entiendo su expresión? What’s the role of le there?

You may hear sentences like:

No le entiendo. – I don’t understand him/her/you (formal).

Here, le is an indirect object pronoun referring to the person.

But combining le with su expresión in the same clause (like No le entiendo su expresión) is not standard in Spain and sounds awkward or non‑native to many speakers.

More natural options are:

No entiendo su expresión. – I don’t understand their/your expression.
No le entiendo la expresión (de la cara). – I don’t understand his/her/your (formal) facial expression.

Even there, many would simply drop le and say:

No entiendo la expresión de su cara.

So: No le entiendo su expresión is best avoided; stick to No entiendo su expresión or No entiendo la expresión de su cara.