No entiendo esta frase.

Because entiendo is the first-person singular form of the verb entender in the present tense.

• entender = to understand
• entiendo = I understand

So No entiendo esta frase literally breaks down as:

• No = not
• entiendo = I understand
• esta frase = this sentence / this phrase

Spanish usually conjugates the verb to show who is doing the action, so you do not need a separate word for I.

In Spanish, subject pronouns are often omitted because the verb ending already tells you the subject.

• entiendo already means I understand
• so Yo no entiendo esta frase is also correct, but yo is optional

Including yo can add emphasis, contrast, or clarity:

• Yo no entiendo esta frase, pero él sí.
  I don’t understand this sentence, but he does.

In neutral speech, No entiendo esta frase sounds very natural.

Because entender is a stem-changing verb in the present tense.

It changes from e → ie in most present-tense forms:

• yo entiendo
• tú entiendes
• él/ella entiende
• nosotros entendemos
• vosotros entendéis
• ellos entienden

Notice that nosotros and vosotros do not change the stem.

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

In Spanish, the usual way to make a sentence negative is to put no directly before the conjugated verb.

So:

• Entiendo esta frase. = I understand this sentence.
• No entiendo esta frase. = I do not understand this sentence.

This is much simpler than English, because Spanish does not need a separate helping verb like do.

Here, esta means this.

It is feminine because it matches frase, which is a feminine noun:

• esta frase = this sentence / this phrase

Spanish demonstratives usually agree in gender and number with the noun:

• este libro = this book
• esta frase = this sentence
• estos libros = these books
• estas frases = these sentences

So esta is feminine singular to match frase.

Because in modern Spanish spelling, demonstratives like este, esta, estos, estas normally do not take an accent.

Older materials may show ésta, but today the standard spelling is simply:

• esta

So in No entiendo esta frase, esta without an accent is the normal modern form.

It can cause confusion because it looks like the English word phrase, but in Spanish frase often means sentence in everyday use.

Depending on context, it can mean:

• a sentence
• a phrase
• an expression

So No entiendo esta frase can naturally be translated as:

• I don’t understand this sentence
• I don’t understand this phrase

If someone is pointing to a full line of text, English often prefers sentence.

Yes. Comprender also means to understand, and No comprendo esta frase is correct.

The difference is mainly one of style and usage:

• entender is very common and everyday
• comprender can sound a little more formal or sometimes a bit more like fully grasp

In many situations, they are interchangeable.

The normal word order is:

• No entiendo esta frase

That is the most natural neutral order.

You may sometimes hear variations for emphasis, such as:

• Esta frase no la entiendo.
  This sentence, I don’t understand it.

But for a learner, No entiendo esta frase is the best standard pattern to use.

A broad pronunciation guide would be:

no en-tee-EN-do ES-ta FRA-se

A more detailed approximation:

• no
• en-tien-do
• es-ta
• fra-se

A few notes for Spain Spanish:

• tiendo in entiendo has a clear y-like glide in the ie sound
• s in esta and frase is a normal s sound
• the stress falls on:
  • -tien- in entiendo
  • e- in esta
  • fra- in frase

So the strongest beats are:

no en-TIEN-do ES-ta FRA-se

Yes, but the meaning changes slightly.

• frase is broad and common in everyday language
• oración is more grammatical/technical and often means sentence in a language-learning or grammar context

So:

• No entiendo esta frase = very natural in general use
• No entiendo esta oración = also correct, but may sound more textbook-like

In casual conversation, frase is often the more natural choice.

The plural version is:

No entiendo estas frases.

Changes:

• esta → estas
• frase → frases

So:

• No entiendo esta frase. = I don’t understand this sentence.
• No entiendo estas frases. = I don’t understand these sentences.