Perdón, no entiendo.

All three are possible depending on context.

In Perdón, no entiendo, perdón works a lot like sorry, excuse me, or pardon me in English when you want to soften what you are about to say.

Here it is not really an apology for doing something wrong. It is more a polite way to say:

• Sorry, I don’t understand.
• Excuse me, I don’t understand.
• Pardon, I don’t understand.

In Latin America, perdón is very common in this kind of situation.

The comma separates two parts:

• Perdón = a polite interjection
• no entiendo = the main statement

So the comma shows a pause, like in English:

• Sorry, I don’t understand.

In speech, people often pause slightly after perdón. In casual writing, some people may leave the comma out, but the comma is standard and helpful.

Spanish often leaves out subject pronouns when they are not necessary.

The verb entiendo already tells you the subject is I, because it is the yo form of entender.

So:

• No entiendo = I don’t understand
• Yo no entiendo = also I don’t understand, but with extra emphasis on I

You usually do not need yo unless you want contrast or emphasis, such as:

• Ella entiende, pero yo no entiendo.
  She understands, but I don’t.

In Spanish, negation with no normally goes directly before the conjugated verb.

So:

• Entiendo = I understand
• No entiendo = I do not understand / I don’t understand

This is the normal Spanish word order. English uses do not plus the base verb, but Spanish does not need an extra helping verb here.

Entiendo is the first-person singular present tense form of the verb entender.

That means:

• entender = to understand
• entiendo = I understand

This verb is stem-changing, so the e in the stem changes to ie in many forms:

• yo entiendo
• tú entiendes
• él/ella entiende

But not in nosotros or vosotros:

• nosotros entendemos

So no entiendo literally breaks down as:

• no = not
• entiendo = I understand

Because entender is a stem-changing verb.

In many present-tense forms, the stem e changes to ie:

• entender
• yo entiendo
• tú entiendes
• usted entiende

This is a common pattern in Spanish. Other verbs like this include:

• querer → quiero
• pensar → pienso
• preferir → prefiero

So entiendo is irregular, but it follows a very common Spanish pattern.

A simple approximate pronunciation for an English speaker is:

• pehr-DON, no en-TYEN-do

A few helpful details:

• Perdón has stress on the last syllable because of the written accent: per-DÓN
• entiendo has stress on tien: en-TIEN-do
• The r in perdón is a light Spanish r, not a strong English r
• The d in Spanish is softer than English d, especially between vowels
• The ie in entiendo sounds like yeh or yenn to many English speakers: en-tyen-do

You do not need a perfect accent to be understood; clear stress is more important at first.

The accent mark shows where the stress goes.

Without getting too technical, perdón is stressed on the last syllable:

• per-DÓN

The accent mark is important in writing because it tells you the correct pronunciation.

If you left it off and wrote perdon, native speakers would still probably understand you in context, but it would be spelled incorrectly.

Yes, it is polite and natural.

It is a perfectly good phrase to use in everyday conversation. In many situations, though, learners also use slightly longer versions to sound softer or more specific, for example:

• Perdón, no entiendo bien.
  Sorry, I don’t understand well.
• Perdón, no le entiendo.
  Sorry, I don’t understand you.
• Perdón, no entendí.
  Sorry, I didn’t understand.
• Disculpe, no entiendo.
  Excuse me, I don’t understand.

So your sentence is already polite, but you can adjust it depending on the situation.

Both can mean I don’t understand, and both are correct.

However, no entiendo is generally more common in everyday speech. It sounds very natural and neutral.

No comprendo is also correct, but in many places it can sound a little more formal, more deliberate, or less common in casual conversation.

For most learners, no entiendo is the safest and most natural everyday choice in Latin America.

They are both useful, but they refer to slightly different situations.

• No entiendo = I don’t understand
  This can mean right now, in general, or as the person is speaking.
• No entendí = I didn’t understand
  This is often used when someone already said something and you missed it.

For example:

• Someone is explaining something complicated right now:
  Perdón, no entiendo.
• Someone said one sentence and you want them to repeat it:
  Perdón, no entendí.

In real conversation, both are common.

You can, but there is a more specific option.

No entiendo by itself usually means I don’t understand in a general sense. It may refer to:

• the words
• the explanation
• the situation
• the idea

If you want to make it clear that you do not understand the person speaking, you can say:

• No te entiendo. = I don’t understand you. informal
• No lo entiendo. = I don’t understand it / him.
• No la entiendo. = I don’t understand it / her.
• No le entiendo. = I don’t understand you / him / her. formal or regional usage

So Perdón, no entiendo is fine, but it is broader and less specific.

Yes. These are common alternatives, with a small difference in tone and formality.

• Perdón = very common, neutral, natural
• Disculpa = informal, used with tú
• Disculpe = formal, used with usted

Examples:

• Perdón, no entiendo.
• Disculpa, no entiendo.
• Disculpe, no entiendo.

In much of Latin America, perdón is especially useful because it works well in many situations without making you think too much about formality.