Mi abuelo cuenta su historia en voz baja antes de dormir.

Cuenta is the third person singular of the verb contar in the present tense.

• contar has two main meanings:
  • to tell / to relate (a story, an anecdote, etc.)
    • Mi abuelo cuenta su historia. = My grandfather tells his story.
  • to count (numbers)
    • Mi abuelo cuenta hasta diez. = My grandfather counts to ten.

In this sentence it clearly means to tell. The full form is:

• él cuenta = he tells / he is telling

Spanish has several verbs that translate as to say / to tell / to speak, but they’re used differently:

• contar = to tell / narrate a story or series of events
  
  • contar una historia, contar un chiste (tell a story / a joke)
• decir = to say / to tell (a specific piece of information or words)
  
  • decir la verdad, decir algo, decir una frase
• hablar = to speak / to talk (more general)
  
  • hablar de política, hablar con mi abuelo

Since the sentence is about narrating a story, contar (→ cuenta) is the most natural verb.

Cuenta is present tense: él cuenta = he tells / he is telling.

In Spanish, the simple present can express:

• a habitual action: something that happens regularly
  
  • In this context, it likely means:
    My grandfather (usually) tells his story in a low voice before going to sleep.
• or an action happening right now (depending on context).

So the sentence can describe a routine (most natural reading) or a current situation; context would decide.

• su historia = his story (or her/their story, depending on context)
• una historia = a story

Using su tells us it is a specific story belonging to someone, presumably the grandfather. The sentence emphasizes his own story, not just any random story he might tell.

If you want to make it very clear that it is his own personal story, you can also say:

• Mi abuelo cuenta su propia historia… = My grandfather tells his own story…

Yes, su is ambiguous by itself:

• su historia can mean his story, her story, their story, or your (formal) story.

In this sentence, context (the presence of mi abuelo) makes it most natural to understand it as his story.

If you need to remove ambiguity, you can say:

• la historia de mi abuelo = my grandfather’s story
• su historia, la de mi abuelo = his story, my grandfather’s

Literally:

• en voz baja = in a low voice

Idiomatic meaning:

• quietly, softly, in a low voice, not loudly

It is a very common fixed expression in Spanish. Some related examples:

• Habla en voz baja. = Speak quietly.
• El niño lee en voz alta. = The child reads aloud / in a loud voice.

Spanish often prefers prepositional phrases instead of adverbs in -mente to describe how someone speaks.

• Natural: en voz baja, en voz alta, en silencio
• Very rare / odd: bajamente (almost never used in this sense)

So en voz baja is the normal, idiomatic way to say in a low voice / softly.

Spanish uses two different patterns:

1. Same subject for both actions → antes de + infinitive

   • Mi abuelo cuenta su historia antes de dormir.
     The person who tells and sleeps is the same (the grandfather), so we use:
     • antes de + dormir (infinitive)

2. Different subjects → antes de que + subjunctive

   • Mi abuelo cuenta su historia antes de que nosotros durmamos.
     (he tells, we sleep)

So antes de dormir is correct because both actions refer to the grandfather.

Yes, very similar.

• antes de dormir literally: before sleeping
• Natural English equivalent: before going to sleep or before he goes to sleep

Both express the same idea of what happens right before he falls asleep.

Spanish usually omits subject pronouns because the verb ending already shows the subject.

• cuenta already implies he / she / it tells.
• Mi abuelo makes the subject clear, so él is not needed.

You can say:

• Él cuenta su historia…, but it often sounds redundant here.
  Subject pronouns (yo, tú, él, nosotros, etc.) are normally used only for emphasis or contrast:

• Él cuenta su historia, no yo. = He tells his story, not me.

Yes, you can move en voz baja and su historia around without changing the basic meaning. All of these are grammatical:

• Mi abuelo cuenta su historia en voz baja antes de dormir.
• Mi abuelo cuenta en voz baja su historia antes de dormir.
• Mi abuelo, en voz baja, cuenta su historia antes de dormir. (more written / dramatic)

The most neutral and common in everyday speech is probably the original:

• Mi abuelo cuenta su historia en voz baja antes de dormir.

Yes, but the structure changes:

• Mi abuelo cuenta su historia antes de dormirse.

  • dormirse (reflexive) = to fall asleep
  • Focus on the moment of falling asleep.

• Mi abuelo cuenta su historia antes de dormir.

  • dormir (non‑reflexive) = to sleep (in general)
  • In practice, here it also means before he goes to sleep / before he falls asleep, and is very natural.

Both are acceptable in this context; antes de dormir is slightly more common and simpler.