Перед ужином я разолью чай по чашкам.

Because перед normally takes the instrumental case.

So:

• ужин = dinner
• перед ужином = before dinner

This is true both in time and space:

• перед ужином = before dinner
• перед домом = in front of the house

For a native English speaker, it helps to learn перед + instrumental as a fixed pattern.

Разолью is future tense.

More specifically, it is the simple future of a perfective verb: разлить.

In Russian:

• perfective verbs form a simple future
• imperfective verbs use быть
  • infinitive for future

So:

• я разолью = I will pour out / I’ll pour out
• я буду разливать = I will be pouring / I will pour repeatedly

Here, разолью suggests a single completed action in the future.

This verb is part of the лить / разлить pattern, which has an irregular-looking conjugation.

Compare:

• лить → лью
• разлить → разолью
• налить → налью

So разолью is just the normal 1st person singular future form of разлить.

This is one of those verb patterns that is best learned as a set:

• я разолью
• ты разольёшь
• он/она разольёт
• мы разольём
• вы разольёте
• они разольют

The prefix раз- often gives the idea of spreading out, distributing, or dividing among several places/containers.

So:

• лить = to pour
• налить = to pour into, fill
• разлить = to pour out into multiple containers, distribute by pouring

That fits well with по чашкам, because the tea is being poured into several cups.

Чай is the direct object, so it is in the accusative case.

But for inanimate masculine singular nouns, the accusative usually looks the same as the nominative.

So:

• nominative: чай
• accusative: чай

That is why there is no visible change.

Чашкам is dative plural.

The phrase по чашкам is a very common Russian pattern meaning something like:

• into the cups
• among the cups
• cup by cup

With verbs of distribution, по + dative often means that something is being divided up among multiple recipients or containers.

So:

• разолью чай по чашкам = I’ll pour the tea into the cups, distributing it among them

This is more distributive than just a simple direction phrase.

Yes, в чашки is possible, but the nuance is a little different.

• в чашки focuses more on the direction: into the cups
• по чашкам focuses more on distribution among the cups

With разлить, по чашкам is especially natural because the verb itself already suggests dividing the liquid among several containers.

So in this sentence, по чашкам is the more idiomatic choice.

Yes. Russian often omits subject pronouns when the verb ending already makes the subject clear.

So both are possible:

• Перед ужином я разолью чай по чашкам.
• Перед ужином разолью чай по чашкам.

Including я can add:

• clarity
• emphasis
• contrast

For example, я might be used if the speaker wants to stress I will do it, not someone else.

No. Russian word order is fairly flexible.

This sentence starts with Перед ужином to set the time frame first:

• Перед ужином я разолью чай по чашкам.

Other orders are possible, for example:

• Я разолью чай по чашкам перед ужином.
• Чай я разолью по чашкам перед ужином.

The basic meaning stays similar, but the focus changes.

In your original sentence, the speaker is naturally foregrounding when the action will happen.

Not necessarily. It usually means before dinner, but it does not always imply right at the last possible moment.

If you want to make it more explicit, you could say:

• прямо перед ужином = right before dinner
• незадолго до ужина = shortly before dinner

So Перед ужином is a normal, neutral way to say before dinner.

Usually no. Разолью is perfective, so it refers to one completed future action.

If you mean a repeated or habitual action, Russian normally uses the imperfective:

• Перед ужином я разливаю чай по чашкам.
  = Before dinner, I pour tea into cups / I usually pour tea into cups before dinner.

If you mean future process rather than one completed act:

• Перед ужином я буду разливать чай по чашкам.

So:

• разолью = one future completed act
• разливаю = habitual/present repeated action
• буду разливать = future imperfective, focusing on process or repeated action

Yes, it is natural.

It sounds like something a person could really say, especially if they are talking about preparing tea before a meal.

What makes it natural is the combination:

• перед ужином = normal time expression
• разолью = appropriate perfective future
• чай = direct object
• по чашкам = very natural with the idea of distributing tea among several cups

So this is a good example of everyday Russian grammar and word choice.