Когда туча ушла дальше на восток, радуга стала ярче.

Because Когда туча ушла дальше на восток is a subordinate clause introduced by когда (when). In Russian, subordinate clauses are normally separated from the main clause with a comma.

So the structure is:

• Когда туча ушла дальше на восток, = When the cloud moved farther east
• радуга стала ярче = the rainbow became brighter

Russian uses this comma whether the когда clause comes first or second:

• Когда туча ушла дальше на восток, радуга стала ярче.
• Радуга стала ярче, когда туча ушла дальше на восток.

Ушла is the past tense of уйти, which is a perfective verb. It shows a completed movement: the cloud went away / moved off.

By contrast, шла is from идти, an imperfective verb, and would focus more on the process of going, not the completed result.

Here, the sentence is about a completed change in the situation:

• the cloud moved away farther east
• then the rainbow became brighter

So ушла fits well because the cloud’s movement is treated as a completed event.

Also, уйти often has the sense of go away / leave, which works nicely for a cloud moving off.

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

The subject here is туча, which is feminine singular, so the verb is feminine singular too:

• masculine: ушёл
• feminine: ушла
• neuter: ушло
• plural: ушли

The same thing happens later in the sentence:

• радуга стала ярче

Радуга is also feminine, so стала is feminine singular.

Both can refer to something in the sky, but they are not exactly the same.

• облако = cloud in the general sense
• туча = a dark rain cloud, storm cloud, or a heavy cloud mass

So туча is more specific and more dramatic. It suggests the kind of cloud that could block sunlight and affect how bright a rainbow looks.

That makes it a very natural choice in this sentence.

It means farther east or further to the east.

Breakdown:

• дальше = farther / further
• на восток = to the east / eastward

Together, they describe movement in an eastern direction, with the idea that the cloud went even farther that way.

So it is not just east, but farther east than before.

Because this sentence describes movement toward a direction, not location.

Russian often uses:

• на + accusative for direction
• на + prepositional for location

Here:

• на восток = to the east / eastward → direction
• на востоке = in the east / in the eastern area → location

Since the cloud is moving, Russian uses восток in the accusative form. For this noun, the accusative looks the same as the nominative: восток.

Yes, but it would sound a little different.

• на восток usually means eastward / to the east as a direction of movement
• к востоку often means to the east of something, or eastward in a slightly more relational or geographic way

In this sentence, ушла дальше на восток sounds very natural because it focuses on the cloud moving off in an easterly direction.

So на восток is the straightforward directional choice.

Because стала ярче means became brighter, showing a change of state.

• стала ярче = it changed and became brighter
• была ярче = it was brighter

The sentence describes a result of the cloud moving away: the rainbow changed and got brighter. So стать + comparative is exactly the pattern Russian likes here.

This is a very common structure:

• стало лучше = it became better
• стало легче = it became easier
• стал сильнее = he became stronger

Because after стать in this meaning, Russian commonly uses the comparative form.

• яркий = bright
• ярче = brighter

So:

• радуга стала ярче = the rainbow became brighter

You could say стала более яркой, but that is less natural here and sounds more formal or heavier in style.

The simple comparative ярче is the normal, idiomatic choice.

In form, ярче is the comparative form connected to яркий. In this sentence, it functions as a predicative comparative after стала.

For a learner, the easiest way to think about it is:

• яркий = bright
• ярче = brighter

And after стать, Russian often uses this comparative directly:

• Он стал выше. = He became taller.
• Она стала спокойнее. = She became calmer.
• Радуга стала ярче. = The rainbow became brighter.

Its basic meaning is when, but in context it can imply something close to once or after.

Because both verbs describe completed past events:

• туча ушла
• радуга стала

the sentence naturally suggests a sequence: once the cloud had moved farther east, the rainbow became brighter.

So the core meaning is still when, but the completed actions make the timing feel a bit like once that happened.

Yes. Russian word order is flexible.

A very natural alternative is:

• Радуга стала ярче, когда туча ушла дальше на восток.

This means the same thing. The difference is mainly in emphasis:

• Когда туча ушла дальше на восток, радуга стала ярче.
  Starts with the time/background situation.

• Радуга стала ярче, когда туча ушла дальше на восток.
  Starts with the main result: the rainbow became brighter.

Both are correct.

Yes:

• Когда = when
• туча = cloud / storm cloud; subject of the subordinate clause
• ушла = went away / moved off; past, feminine singular, perfective
• дальше = farther / further
• на восток = to the east / eastward
• радуга = rainbow; subject of the main clause
• стала = became; past, feminine singular
• ярче = brighter

So the structure is:

When [the storm cloud moved farther east], [the rainbow became brighter].

Russian does not have a separate tense that works exactly like the English past perfect in normal everyday use. It usually relies on context, aspect, and clause structure instead.

So Russian simply says:

• Когда туча ушла дальше на восток, радуга стала ярче.

Even though English might sometimes say When the cloud had moved farther east, the rainbow became brighter, Russian normally just uses the past tense and lets the sequence of events be understood from the context.

The perfective verbs help make that sequence clear.