По мере того как вафли остывали, сироп становился гуще.

По мере того как means as, while, or more literally in proportion as. It is used when two processes develop gradually together.

In this sentence:

• По мере того как вафли остывали... = As the waffles were cooling...
• ...сироп становился гуще = ...the syrup was becoming thicker

So the idea is not just that one thing happened during another, but that the change in one thing went along with the change in the other.

It is a common pattern for gradual change:

• По мере того как темнело, становилось холоднее. = As it got darker, it got colder.

Because по мере того как вафли остывали is a subordinate clause, and Russian normally separates subordinate clauses from the main clause with a comma.

So the structure is:

• По мере того как вафли остывали, = subordinate clause
• сироп становился гуще. = main clause

This is very similar to English:

• As the waffles cooled, the syrup became thicker.

Because its subject is вафли, which is plural.

• вафли = waffles
• остывали = were cooling / cooled in the past plural form

In the Russian past tense, verbs agree with the subject in number, and in the singular they also agree in gender.

Compare:

• вафли остывали = the waffles were cooling
• вафля остывала = the waffle was cooling

Because its subject is сироп, which is singular masculine.

• сироп = a masculine noun
• становился = past tense, masculine singular

Russian past tense forms show:

• singular masculine: становился
• singular feminine: становилась
• singular neuter: становилось
• plural: становились

So:

• сироп становился гуще = the syrup was becoming thicker
• глазурь становилась гуще = the icing was becoming thicker
  because глазурь is feminine

Because the sentence describes a gradual process, not a single completed event.

• остывать = imperfective, to cool down / to be cooling
• past plural: остывали

With по мере того как, Russian often uses the imperfective because the focus is on something unfolding over time.

Compare:

• вафли остывали = the waffles were cooling / cooled gradually
• вафли остыли = the waffles cooled down / became cool as a completed result

Here the idea is: while they were gradually cooling, the syrup was gradually getting thicker. That is why imperfective fits naturally.

Because становился means was becoming or was getting, which shows change over time.

• сироп был густой = the syrup was thick
  This just describes its state.
• сироп становился гуще = the syrup was becoming thicker
  This shows a process of change.

Since the sentence is about one thing changing as another thing changes, становился is the natural choice.

Гуще is the comparative form of густой and means thicker.

So:

• густой = thick
• гуще = thicker

After становиться, Russian can use:

1. an adjective in a form that describes the new quality
2. a comparative, if the meaning is more X

Here the meaning is specifically becoming thicker, so the comparative гуще is exactly right.

Compare:

• сироп становился густым = the syrup was becoming thick
• сироп становился гуще = the syrup was becoming thicker

The second one emphasizes change relative to an earlier state.

It comes from the adjective густой, but here it functions as a comparative predicate meaning thicker.

For learners, the most useful way to think about it is:

• густой = adjective, thick
• гуще = comparative form, thicker

In sentences like this, Russian often uses the comparative directly after verbs like становиться:

• становиться лучше = to become better
• становиться хуже = to become worse
• становиться гуще = to become thicker

So even though English may make you want a full adjective phrase, Russian often just uses the comparative form.

Not exactly. Когда means when, but it does not by itself express the idea of gradual parallel change.

Compare:

• Когда вафли остывали, сироп становился гуще. = When the waffles were cooling, the syrup was getting thicker.
• По мере того как вафли остывали, сироп становился гуще. = As the waffles cooled, the syrup became thicker / The more the waffles cooled, the thicker the syrup became.

The version with по мере того как more clearly connects the two ongoing changes.

So когда is possible in some contexts, but it is weaker and less precise here.

Because the whole sentence describes a situation in the past.

Both actions were happening at the same time in that past situation:

• вафли остывали = the waffles were cooling
• сироп становился гуще = the syrup was becoming thicker

Russian often uses the past tense this way for two simultaneous background processes.

If you wanted the same idea in the present, you could say:

• По мере того как вафли остывают, сироп становится гуще. = As the waffles cool, the syrup gets thicker.

Yes, Russian word order is flexible, although the original order is the most neutral.

Original:

• По мере того как вафли остывали, сироп становился гуще.

You could also say:

• Сироп становился гуще по мере того, как вафли остывали.

This still means the same thing, but it puts the main clause first and sounds a bit more written or carefully structured.

The original version is very natural because it starts with the time/condition clause and then gives the result happening alongside it.

A few useful points:

• вафли = waffles
  • plural form
  • singular: вафля
• остывать = to cool down
  • not to cool something, but to become cool
• сироп = syrup
• густой = thick
• гуще = thicker

A very common pattern in Russian is:

• X остывает / нагревается / темнеет
• Y становится лучше / хуже / гуще / светлее

So this sentence is a good example of how Russian describes natural gradual changes.