Mañana voy a retomar la lección en la biblioteca.

Yes, you can move mañana around. It’s an adverb of time, and Spanish is flexible with those.

Possible word orders:

• Mañana voy a retomar la lección en la biblioteca.
• Voy a retomar la lección mañana en la biblioteca.
• Voy a retomar mañana la lección en la biblioteca.

All are correct. Putting mañana at the beginning slightly emphasizes when you will do it. In everyday speech, the first and second options are the most common.

A comma after Mañana is optional:

• Mañana, voy a retomar… is acceptable, but for a short sentence it’s usually written without a comma.

Both talk about the future, but there’s a nuance:

• Voy a retomar la lección…

  • Form: ir (conjugated) + a + infinitive
  • Very common in spoken Spanish (especially in Latin America).
  • Often used for plans or intentions: I’m going to resume…

• Retomaré la lección…

  • Simple future tense.
  • Feels a bit more formal or “written”.
  • Can sound slightly more distant or less “right now I’ve decided this”.

In Latin American everyday speech, voy a retomar is more natural than retomaré in most contexts.

• tomar = to take, to drink, to take (a class, a bus, a test), etc.
• retomar = re-take, pick up again, resume something you had started before.

So:

• tomar la lección → to take the lesson (for the first time, or in general).
• retomar la lección → to continue the lesson, to go back to it after a pause.

In this sentence, retomar implies you already started that lesson earlier, stopped, and now you’re going back to it.

Spanish normally doesn’t use the present progressive (estar + gerundio) to talk about the future the way English does.

Compare:

• English: I’m restarting the lesson tomorrow.
• Spanish:
  
  • Voy a retomar la lección mañana. (ir a + infinitive)
  • Retomaré la lección mañana. (simple future)

Estoy retomando la lección means I’m (currently) resuming the lesson right now, not tomorrow.

So for future plans, Spanish prefers:

• ir a + infinitive (very frequent), or
• simple future (retomaré), depending on style/level of formality.

Spanish uses definite articles more than English:

• la lección = the lesson.
  It often refers to a specific, known lesson (for example, the one you’re studying in your course/book).

You could say:

• mi lección (my lesson) — focusing more on possession: your homework, your part of the course.
• una lección (a lesson) — a non-specific lesson, just some lesson.

In a typical study context, la lección is natural because both speaker and listener know which lesson is being talked about (for example, lesson 3 of the textbook).

Both are feminine:

• la lección
• la biblioteca

Clues:

• Nouns ending in -ción (lección, nación, información) are almost always feminine:
  
  • la lección, la nación, la información.
• Nouns ending in -teca (biblioteca, discoteca, videoteca) are also feminine:
  
  • la biblioteca, la discoteca.

So you say la lección, una lección, esta lección; and la biblioteca, una biblioteca, esa biblioteca.

• en la biblioteca = in/at the library (location where something happens).
• a la biblioteca = to the library (movement/direction).

In your sentence, you’re saying where you will resume the lesson (location), not where you’re going:

• Mañana voy a retomar la lección en la biblioteca.
  Tomorrow I’m going to resume the lesson *in/at the library.*

If you wanted to emphasize going there, you could say:

• Mañana voy a ir a la biblioteca para retomar la lección.
  Tomorrow I’m going to go to the library to resume the lesson.

Yes, that’s grammatically correct:

• Mañana en la biblioteca voy a retomar la lección.

Spanish allows quite a bit of movement for adverbs and prepositional phrases. Different orders give slight emphasis changes:

• Mañana voy a retomar la lección en la biblioteca.
  Neutral, very natural.
• En la biblioteca mañana voy a retomar la lección.
  More emphasis on “at the library” (sounds a bit more formal or styled).
• Mañana en la biblioteca voy a retomar la lección.
  Emphasizes both time and place first. Still okay, though slightly less common in everyday speech.

All of these are understandable and grammatically fine.

On its own, mañana almost always means tomorrow.

To mean morning, Spanish usually adds something:

• por la mañana = in the morning
• en la mañana (very common in much of Latin America) = in the morning
• esta mañana = this morning

So:

• Mañana voy a retomar la lección… → Tomorrow I’m going to resume the lesson…
• Por la mañana voy a retomar la lección… → In the morning I’m going to resume the lesson…

They’re close in meaning, but not identical:

• retomar la lección

  • More compact.
  • Means to resume/pick up the lesson again (any kind of work with that lesson).

• volver a estudiar la lección

  • Literally “to study the lesson again”.
  • Emphasizes the action of studying once more.

You could also say:

• volver a retomar la lección (to resume it again), but that’s usually unnecessary repetition unless you need that emphasis.

They’re different:

• la biblioteca = library (place where you borrow or consult books).
• la librería = bookstore (place where you buy books).

So in your sentence, en la biblioteca clearly means in the library, not in a bookstore.

Approximate pronunciation (Latin American):

• Mañana → [mah-NYAH-nah]

  • ñ like the “ny” in canyon.
  • Stress on the second syllable: ma-ÑA-na.

• voy → [boy] (like English boy).

• a → [ah].

• retomar → [reh-toh-MAR]

  • Stress on the last syllable: reto-MAR.

• la → [lah].

• lección → [lek-SYON]

  • cc + i → sounds like “ksy” here.
  • The accent on ó means the stress is on the last syllable: lec-CIÓN.
  • Final -ón sounds like “ohn”.

• en → [en].

• la biblioteca → [lah bee-blyoh-TEH-kah] or [lah bee-blee-oh-TEH-kah]

  • In most Latin American accents, b and v sound the same (like English “b”).
  • Stress on te: biblio-TE-ca.

Ir (to go) is irregular. Present tense forms:

• yo voy
• tú vas
• él/ella/usted va
• nosotros vamos
• ustedes/ellos van

In your sentence, the subject is yo (I), even though it’s not written, so you need voy:

• (Yo) voy a retomar la lección…
  I am going to resume the lesson…

If the subject were someone else:

• Mañana vas a retomar la lección… (you, informal singular)
• Mañana va a retomar la lección… (he/she/you formal)
• Mañana vamos a retomar la lección… (we)
• Mañana van a retomar la lección… (they / you all)