Я боялась, что кактус засохнет на солнце, но он оказался крепче, чем я думала.

Because the verb is in the past tense, and in Russian past-tense verbs agree with the speaker’s gender in the singular.

• Я боялась = I was afraid / I feared said by a woman
• Я боялся = the same thing said by a man

So this sentence suggests that the speaker is female.

Боялась is the past tense of бояться, which means to be afraid or to fear.

In this sentence, it does not mean a sudden moment of fear. It suggests an ongoing worry:

• Я боялась, что... = I was afraid that... / I worried that...

That fits the idea of being concerned the cactus might dry out.

Что introduces a clause meaning that:

• Я боялась, что кактус засохнет... = I was afraid that the cactus would dry out...

This is very common in Russian after verbs of thinking, saying, knowing, fearing, etc.

Compare:

• Я думала, что... = I thought that...
• Я знала, что... = I knew that...
• Я боялась, что... = I was afraid that...

Russian does not do the same kind of tense backshifting that English often does.

English says:

• I was afraid that the cactus would dry out

Russian says literally:

• I was afraid that the cactus will dry out

So засохнет is a future form, even though the main verb is in the past.

Also, засохнет is perfective future, which is important here: it refers to the cactus reaching the result of drying up.

Засохнет comes from засохнуть, a perfective verb meaning:

• to dry out
• to shrivel up
• to wither from lack of moisture

It is especially natural for plants.

So for a cactus, засохнуть suggests not just becoming dry, but drying up in a harmful way.

Related forms:

• засохнуть = perfective
• засыхать = imperfective

In this expression, на солнце means in the sun or in direct sunlight.

This is just how Russian commonly says it:

• сидеть на солнце = to sit in the sun
• оставить цветок на солнце = to leave a plant in the sun

So although на often means on, here the whole phrase is idiomatic and simply means exposure to sunlight.

Because Russian uses grammatical gender.

• кактус is a masculine noun
• so the pronoun is он = he grammatically

In English, objects are usually it, but in Russian nouns are masculine, feminine, or neuter, and pronouns match that gender.

So:

• кактус ... он
• книга ... она
• окно ... оно

This does not mean the cactus is male in a biological sense.

Оказался comes from оказаться, which means:

• to turn out
• to prove to be
• to end up being

So:

• но он оказался крепче = but it turned out to be tougher / hardier

It is masculine singular past, agreeing with кактус.

This verb is very common when reality is different from what someone expected:

• Он оказался прав. = He turned out to be right.
• Книга оказалась интересной. = The book turned out to be interesting.

Крепче is the comparative form of крепкий.

• крепкий = strong, sturdy, tough
• крепче = stronger, sturdier, tougher

So:

• он оказался крепче = it turned out to be tougher

Russian often uses simple comparative forms like this instead of a word equivalent to more:

• быстрый → быстрее = fast → faster
• сильный → сильнее = strong → stronger
• крепкий → крепче = sturdy → sturdier

Literally, yes, крепче is stronger or sturdier. But for a cactus, the more natural English idea is something like:

• hardier
• tougher
• more resilient

So the sentence means the cactus handled the sun better than the speaker expected.

This is the standard way to make a comparison:

• крепче, чем я думала = tougher than I thought

Here:

• чем = than
• я думала = I thought (female speaker again)

So the structure is:

• [comparative] + чем + clause

Examples:

• лучше, чем я ожидал = better than I expected
• труднее, чем кажется = harder than it seems

Думала is from думать, an imperfective verb meaning to think.

That works well here because it refers to the speaker’s earlier mental state, not one completed single thought. English also naturally says:

• than I thought

not usually than I finished thinking once

So думала is the normal choice.

Also, because the speaker is female, it is думала rather than думал.

It is fairly neutral and natural.

• Я боялась, что кактус засохнет на солнце, но он оказался крепче, чем я думала.

This word order clearly presents:

1. the fear
2. the possible result
3. the contrast
4. the actual outcome

Russian word order is flexible, but this version sounds normal and straightforward. You could move things around for emphasis, but this is a very standard way to say it.

They are there for standard Russian punctuation rules.

1. Я боялась, что...
   A comma is used before что because it introduces a subordinate clause.

2. ..., но он оказался...
   A comma is used before но because it joins two clauses with but.

3. ..., чем я думала
   A comma is used before чем when it introduces the comparison clause than I thought.

So all the commas here are expected and normal.