Yo sí comprendo la lección hoy.

In Yo sí comprendo la lección hoy, sí is not the standalone answer “yes.”

Here it’s an emphatic adverb meaning something like:

• “I do understand the lesson today.”
• “I really / actually / indeed understand the lesson today.”

It often implies contrast:

• Yo sí comprendo la lección hoy.
  = I (do) understand the lesson today (even if others don’t / even if I didn’t before).

Note the accent:

• sí (with accent) = “yes,” or emphatic “do/indeed.”
• si (no accent) = “if.”

Yes, Yo comprendo la lección hoy is perfectly correct.

The difference is nuance:

• Yo comprendo la lección hoy.
  = Neutral statement: I understand the lesson today.

• Yo sí comprendo la lección hoy.
  = Emphatic / contrastive: I *do understand the lesson today*, maybe contrary to what someone thinks, or compared to another time/person.

So sí is optional, but it adds emphasis.

You can absolutely drop yo:

• Sí comprendo la lección hoy.
• Comprendo la lección hoy.

Spanish usually omits subject pronouns because the verb ending (comprendo) already shows the subject is yo.

When is yo used?

• For emphasis or contrast:
  Yo sí comprendo la lección hoy, pero ellos no.
  I do understand the lesson today, but they don’t.

• To clarify in a longer context.

Grammatically, all of these are correct:

• Yo sí comprendo la lección hoy.
• Sí comprendo la lección hoy.
• Yo comprendo la lección hoy.
• Comprendo la lección hoy.

In practice, in Latin American Spanish:

• comprender ≈ entender
  Both usually mean “to understand.”

General tendencies (not strict rules):

• entender is more common in everyday, informal speech.
• comprender can sound slightly more formal, or closer to “to comprehend,” “to grasp deeply.”

Examples:

• No entiendo la lección. / No comprendo la lección.
  I don’t understand the lesson.

• Ahora comprendo lo que quieres decir.
  Now I (really) comprehend what you mean.

But most of the time, learners can treat them as synonyms.

Because lección is a feminine noun in Spanish.

• It uses the article la: la lección.
• Many nouns ending in -ción are feminine:
  • la nación (nation)
  • la canción (song)
  • la información (information)

Also note the stress and accent: lección is stressed on -cción and has an accent on ó.

Spanish uses definite articles (el, la, los, las) more often than English.

In comprendo la lección, la points to a specific lesson both the speaker and listener know about (e.g., today’s lesson in class).

Usually you cannot omit the article here:

• ❌ Comprendo lección hoy. (incorrect)
• ✅ Comprendo la lección hoy.

You might omit it only if you change the structure, for example:

• Comprendo la lección de hoy. (I understand today’s lesson.)
• Comprendo esta lección. (I understand this lesson.)

But some determiner (article, esta, esa, etc.) is normally needed.

Hoy (today) is quite flexible in position. All of these are grammatical:

• Hoy sí comprendo la lección.
• Hoy comprendo la lección.
• Yo sí comprendo la lección hoy.
• Comprendo la lección hoy.

The basic meaning (“today I understand the lesson”) stays the same.
Word order mainly affects emphasis:

• Hoy sí comprendo la lección.
  Emphasizes today (maybe: I didn’t understand it before, but today I do).

• Yo sí comprendo la lección hoy.
  Emphasizes I (yo) and sí (I do), with hoy giving time info at the end.

Because comprendo is the first-person singular form of comprender in the present tense.

Present tense of comprender:

• yo comprendo – I understand
• tú comprendes – you understand
• él / ella / usted comprende – he/she/you (formal) understand(s)
• nosotros comprendemos – we understand
• ellos / ellas / ustedes comprenden – they / you all understand

So:

• Yo comprendo = I understand
• Él comprende = He understands

Comprendo is present simple in Spanish, but it often covers both:

• English “I understand”
• English “I am understanding” (when English uses progressive)

Spanish uses the progressive (estar + gerund) less often than English, and almost never with verbs like “understand”:

• ❌ Estoy comprendiendo la lección hoy (grammatical but odd in most contexts)
• ✅ Comprendo la lección hoy (normal)

So comprendo is the natural way to say “I understand / I am understanding” here.

Yes, but the tense and meaning change:

• Comprendo la lección hoy.
  Present: I understand the lesson today (right now / at this point).

• Entendí la lección hoy.
  Preterite: I understood the lesson today (at some point earlier today, and that action is complete).

Use the present (comprendo / entiendo) when focusing on your current state of understanding.
Use the preterite (entendí / comprendí) to talk about when you came to understand it (a finished event earlier today).

Basic pronunciation and stress:

• comprendo → com–pren–do
  Stress on pren.

• la → just like English “la” in “la-la,” very short.

• lección → lec–ción
  Stress on ción. The cc is pronounced like k + s: approx. lek-syón.

So the main stressed syllables are pren and ción:
comPRENdo la lecCIÓN.

Yes, that’s correct Spanish:

• Yo sí la comprendo hoy.
  = I do understand it today.

Here:

• la is a direct object pronoun replacing la lección (feminine, singular).
• Word order is: (subject) + sí + object pronoun + verb.

Both are fine:

• Yo sí comprendo la lección hoy. (explicit noun)
• Yo sí la comprendo hoy. (pronoun replacing a known noun)

You’d typically use la once “the lesson” is already clear from the context.