Когда я снова полила почву, орхидея выглядела лучше, а стебель стал крепче.

Because полила is the perfective past form of полить. It presents the watering as a completed single action: when I watered the soil again.

Поливала would be the imperfective form and would sound more like:

• I was watering
• I used to water
• I watered repeatedly / as a process

In this sentence, the speaker is talking about one completed event that led to a result, so полила fits better.

In the Russian past tense, singular verbs agree with the subject's gender.

So:

• masculine: полил, выглядел
• feminine: полила, выглядела
• neuter: полило, выглядело

Here:

• я полила suggests the speaker is female
• орхидея выглядела is feminine because орхидея is a feminine noun

If the speaker were male, he would say Когда я снова полил почву...

Because почву is the accusative case of почва.

The verb полить takes a direct object: you water something.
So:

• nominative: почва = soil
• accusative: почву = soil as the object of the action

Compare:

• Почва сухая. = The soil is dry.
• Я полила почву. = I watered the soil.

Because Когда я снова полила почву is a subordinate clause introduced by когда.

Russian normally separates subordinate clauses with a comma:

• Когда я пришёл, он спал.
• Если будет дождь, мы останемся дома.

So here:

• Когда я снова полила почву, ...

The comma marks the boundary between when I watered the soil again and the main clause.

Снова means again.

Here it modifies полила:

• Когда я снова полила почву... = When I watered the soil again...

Its position is fairly flexible, but it usually stays near the verb:

• Когда я снова полила почву...
• Когда я полила почву снова... — possible, but less neutral here

The most natural place is usually before the verb, as in the original sentence.

Because лучше is the correct comparative form used with выглядеть.

• выглядеть лучше = to look better

A few important points:

• лучше already means better, so более лучше is incorrect because it would be like saying more better
• лучшая is an adjective meaning best / better in forms like the better one, but it does not fit after выглядела

So:

• орхидея выглядела лучше = the orchid looked better

This is the normal Russian way to say it.

Because стал means became, while был means was.

• стал крепче = became stronger / got sturdier
• был крепче = was stronger

The sentence is describing a change in condition, so стать is the natural verb:

• стебель стал крепче = the stem became stronger

If you used был, it would describe a state rather than a change.

Крепче is the comparative form of крепкий.

So:

• крепкий = strong, sturdy, firm
• крепче = stronger, sturdier

After стать, Russian often uses this short comparative form:

• стал лучше = became better
• стал сильнее = became stronger
• стал крепче = became sturdier

You could also say стал более крепким, but стал крепче is more natural and common.

Both а and и can mean and in some contexts, but they are not the same.

Here а works well because it connects two related statements while also slightly contrasting or setting them side by side:

• the orchid looked better,
• and the stem became stronger

It is like and / while / whereas depending on context.

Using и would sound more like simple addition:

• орхидея выглядела лучше, и стебель стал крепче

That is possible, but а is very natural here because it presents two separate positive results in parallel.

Because they are the subjects of their clauses.

In the main part of the sentence:

• орхидея выглядела лучше — орхидея is the subject
• стебель стал крепче — стебель is the subject

Subjects are normally in the nominative case in Russian.

By contrast, почву is not the subject; it is the direct object of полила, so it is in the accusative.

Literally, когда means when. But in a sentence like this, especially with a perfective verb such as полила, it often implies a sequence close to after:

• Когда я снова полила почву, орхидея выглядела лучше...

This suggests that after the watering was completed, the results were noticeable.

So the basic meaning is still when, but in context English may naturally interpret it as after I watered the soil again.

Not completely. Russian word order is fairly flexible, but the original sentence is neutral and natural.

Original:

• Когда я снова полила почву, орхидея выглядела лучше, а стебель стал крепче.

You could rearrange parts for emphasis, for example:

• Орхидея выглядела лучше, когда я снова полила почву...
• Когда я почву снова полила... — possible, but marked and less neutral

Russian word order often changes the focus or emphasis, not the basic grammatical meaning. For a learner, the original version is the safest and most natural model.