Один кактус почти засох, а второй кактус всё ещё зелёный.

Because один ... второй ... is a very common way to contrast two things: one ... the other / the second ....

Here, один кактус means one cactus, and второй кактус means the second / the other cactus.

Using первый ... второй ... is also possible, but it sounds more like you are formally numbering them as first and second. In everyday comparison, один ... второй ... often sounds more natural.

It primarily means second, but in a two-item contrast it often works like the other one.

So in this sentence, второй кактус is naturally understood as the other cactus or the second cactus. Russian often uses второй where English might prefer the other.

А is very often used to contrast two parallel facts.

Here the structure is:

• one cactus is in one condition
• the second cactus is in a different condition

So а fits very well: it means something like whereas, while, or and as for the other one.

• и would only add information: and
• но would sound like a stronger contradiction: but

In this sentence, а is the most natural choice because the speaker is comparing two items side by side.

Because а connects two separate clauses here:

• Один кактус почти засох
• второй кактус всё ещё зелёный

Each part has its own subject and predicate, so Russian normally puts a comma before а.

In the present tense, Russian usually omits the verb to be.

So:

• второй кактус зелёный literally looks like second cactus green
• but it means the second cactus is green

This is completely normal in Russian. English needs is, but Russian usually does not in the present tense.

Compare:

• Кактус зелёный = The cactus is green
• Кактус был зелёный = The cactus was green

In the present tense, the linking verb is usually just left out.

Зелёный is a masculine singular adjective, and it agrees with кактус, which is a masculine singular noun.

So the forms match:

• кактус = masculine singular
• зелёный = masculine singular

This is the normal full adjective form used in sentences like кактус зелёный.

A learner may also notice that Russian sometimes has short adjective forms, such as зелён, but that would sound more literary or less neutral here. Зелёный is the ordinary everyday choice.

Всё ещё means still.

So:

• всё ещё зелёный = still green

This expression emphasizes that the situation continues up to now.

You can think of it as stronger and more explicit than plain ещё in many contexts. In this sentence, всё ещё is very natural because it highlights the contrast: one cactus almost dried up, but the other one remains green.

Засох is the past tense masculine singular form of засохнуть, which means to dry up, to wither, or to dry out completely.

With plants, засохнуть often suggests that the plant has dried out badly, possibly to the point of dying.

So почти засох means almost dried up or almost withered completely.

This fits a cactus well: it suggests that the cactus got very dry and came close to that end state.

Because почти often goes very naturally with a perfective verb when you want to say that something came close to reaching a result.

Here, the result is being dried up. So:

• засох names that completed result
• почти засох means it came close to that result, but did not fully reach it

This is a very common pattern in Russian.

By contrast, почти засыхал would sound much less natural here, because the imperfective focuses on process rather than the near-completion of a result.

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

Since кактус is masculine singular, the verb is also masculine singular:

• masculine: засох
• feminine: засохла
• neuter: засохло
• plural: засохли

So Один кактус почти засох uses засох because кактус is masculine.

No, it is not strictly necessary.

You could also say:

Один кактус почти засох, а второй всё ещё зелёный.

That is probably the more natural concise version in everyday speech.

Repeating кактус is still correct. It can make the contrast a little clearer or more balanced, especially for emphasis or careful explanation.

Not completely. Russian word order is flexible, but this sentence uses a very natural neutral order:

• Один кактус = topic of the first clause
• почти засох = what happened to it
• а второй кактус = contrasting topic
• всё ещё зелёный = its condition

This order is good for a simple comparison. Other word orders are possible, but they would usually change the emphasis.