A medida que pasaba la tarde, mi abuelo se iba cansando en la grada.

Yes. A medida que is a very common fixed expression meaning as, as/while, or more specifically the more/as something gradually happens.

In this sentence, it introduces a parallel change:

• A medida que pasaba la tarde... = As the afternoon went on...

It often appears when two things develop together:

• A medida que estudiaba, entendía más.
  As he studied, he understood more.

So it is not usually translated word-for-word. Think of it as one chunk.

Because Spanish uses the imperfect here to describe something ongoing in the background.

• pasaba = was passing / went on
• pasó = passed as a completed event

In A medida que pasaba la tarde, the afternoon is unfolding gradually, not being presented as a single finished event. That fits the imperfect very naturally.

Compare:

• Pasó la tarde y nos fuimos.
  The afternoon passed and we left.
  This sounds more like a completed sequence.

• A medida que pasaba la tarde...
  As the afternoon went on...
  This focuses on the process.

Yes, la tarde is the subject here.

In this sentence, pasar means something like to pass / to go by:

• pasaba la tarde = the afternoon was passing / the afternoon was going by

Spanish often allows subject placement after the verb, especially when describing events or settings. So this word order is completely normal.

Be careful: pasar la tarde can also mean to spend the afternoon, but that is a different structure.

• Pasé la tarde en casa. = I spent the afternoon at home.
• Pasaba la tarde. = The afternoon was passing.

Ir + gerund adds the idea of gradual change over time.

So:

• se cansaba = he was getting tired / he used to get tired
• se iba cansando = he was gradually getting more and more tired

In this sentence, that gradual feeling matters. The idea is that as the afternoon went on, the grandfather became increasingly tired.

This is a very common use of ir + gerund:

• La gente se iba marchando.
  People were gradually leaving.
• Me iba preocupando cada vez más.
  I was getting more and more worried.

So se iba cansando is more expressive than the plain imperfect.

Because the verb here is cansarse, not just cansar.

• cansar = to tire someone/something out
  
  • El viaje me cansó. = The trip tired me out.
• cansarse = to get tired / to become tired
  
  • Mi abuelo se cansó. = My grandfather got tired.

In your sentence, the grandfather is the one becoming tired, so Spanish uses the pronominal form:

• mi abuelo se iba cansando = my grandfather was gradually getting tired

Yes. Both are correct.

With a verbal structure like ir + gerund, the pronoun can usually go:

1. before the conjugated verb
   
   • se iba cansando
2. attached to the gerund
   
   • iba cansándose

Both mean the same thing. The first version is often a bit more neutral and common in everyday prose, but both are perfectly standard.

If the pronoun is attached to the gerund, remember the written accent:

• cansando → cansándose

It often expresses a process that develops little by little.

That is the key nuance here.

• se está cansando = he is getting tired / happening now
• se iba cansando = he was gradually getting tired
• se fue cansando = he gradually got tired in a completed past sequence

So ir + gerund is especially useful when something changes progressively.

Other examples:

• El cielo iba oscureciéndose.
  The sky was gradually getting darker.
• Nos íbamos acercando al estadio.
  We were gradually getting closer to the stadium.

Because both actions are presented as ongoing, developing, and simultaneous in the past.

The sentence is painting a scene:

• the afternoon was passing
• the grandfather was gradually getting tired

This is a classic use of the imperfect: background description and processes in progress.

If Spanish used preterite forms instead, the sentence would feel more like completed events rather than an unfolding situation.

In Spain, la grada usually means the stands, the bleachers, or the seating area in a stadium, arena, bullring, or similar venue.

So:

• en la grada = in the stands / sitting in the stands

In Peninsular Spanish, this is a very natural word in sports or public-event contexts.

Depending on context, English often prefers the plural:

• en la grada → in the stands

Even though Spanish uses singular here, the English translation may sound better in the plural.

Both can exist, but they are not always used in exactly the same way.

In many contexts:

• la grada can mean the stand/seating section as a collective area
• las gradas can mean the stands/bleachers in a more explicitly plural sense

Spanish often uses the singular for a collective or general place where English might prefer the plural.

So en la grada sounds natural if the speaker is thinking of the seating area as one location. English would still often say in the stands.

Not exactly. They are related, but the nuance changes.

• mientras = while
• a medida que = as / as... progressively / the more...

If you say:

• Mientras pasaba la tarde, mi abuelo se iba cansando...

it sounds less idiomatic and less precise for this kind of gradual parallel development.

A medida que is better because it highlights that one thing changes progressively as the other progresses.

So for this sentence, a medida que is the best choice.

The grammar is general Spanish, but grada in this context feels especially natural in Spain.

A learner might hear other words in other regions depending on the setting, but nothing in the sentence is strange outside Spain. The most notably Spain-flavored element is probably la grada.

The rest:

• a medida que
• pasaba la tarde
• se iba cansando

are all standard Spanish.

Not really. A literal translation would sound unnatural in English.

Spanish and English package these ideas differently. A natural English version is more like:

• As the afternoon went on, my grandfather was getting tired in the stands.
• As the afternoon passed, my grandfather was gradually getting tired in the stands.

So even though iba literally relates to going, in se iba cansando you should not translate ir directly as go. The whole structure means was gradually getting.