Мало того что я забыла полить кактус, так ещё и оставила его без света.

This is a common Russian pattern meaning something like:

• Not only ... , but also ...
• As if that weren’t enough, ...
• To make matters worse, ...

So the sentence has a complaining or frustrated tone. The speaker is saying that one bad thing happened, and then there was an additional bad thing on top of it.

In this sentence:

• Мало того что я забыла полить кактус = Not only did I forget to water the cactus
• так ещё и оставила его без света = I also left it without light / and to make matters worse, I left it without light

The expression often sounds stronger and more emotional than plain не только ..., но и ....

Because the speaker is female.

Russian past tense verbs agree with gender in the singular:

• masculine: забыл, оставил
• feminine: забыла, оставила
• neuter: забыло, оставило
• plural: забыли, оставили

Since the subject is я, the verb form does not itself tell you the person, but it does show the speaker’s gender in the past tense.

So:

• я забыла = I forgot (said by a woman)
• я забыл = I forgot (said by a man)

Because Russian often uses забыть + infinitive to mean to forget to do something.

So:

• забыла полить кактус = forgot to water the cactus

This is different from:

• забыла кактус — this would not work for forgot to water the cactus
• забыла о кактусе = forgot about the cactus

So забыть + infinitive focuses on a forgotten action, not a forgotten object or idea.

This is an aspect question.

• полить = perfective
• поливать = imperfective

Here, полить is used because the speaker means one complete action: to water the cactus once.

With забыть + infinitive, Russian often uses the perfective when talking about forgetting to carry out a single whole action:

• Я забыл закрыть окно. = I forgot to close the window.
• Она забыла позвонить. = She forgot to call.

If you used поливать, it would sound more like forgetting to do it habitually or over a period of time, depending on context. In this sentence, the idea is one specific missed act of watering, so полить is the natural choice.

Кактус is the direct object of полить, so it is in the accusative case.

However, for inanimate masculine singular nouns, the accusative form is usually the same as the nominative form.

So:

• nominative: кактус
• accusative: кактус

That is why the form does not visibly change.

Compare:

• Я вижу стол. = I see the table.
• Я полила кактус. = I watered the cactus.

But with an animate masculine noun, the accusative would usually match the genitive instead.

Его means him or it, depending on context. Here it means it, referring back to кактус.

Since кактус is:

• masculine
• singular

the pronoun used is его.

In this sentence, его is the direct object of оставила, so it is in the accusative. For masculine animate/inanimate and for genitive, его has the same form, so the form itself does not change.

So:

• оставила его = left it

Literally, без света means without light.

Grammatically:

• без is a preposition meaning without
• без requires the genitive case
• свет becomes света in the genitive singular

So:

• свет = light
• без света = without light

In context, оставила его без света means something like:

• left it without light
• left it in the dark
• left it where it wasn’t getting light

For a cactus, this means the plant was not getting the light it needed.

Так ещё и adds emphasis. It makes the second part feel like an extra annoyance or an additional mistake.

Compare the feel:

• ещё и = also / on top of that
• так ещё и = and to make matters worse / and on top of that

So in this sentence, так ещё и strengthens the speaker’s sense of self-criticism or frustration.

It fits especially well after мало того что. Together they create a very natural pattern for piling one problem on top of another.

There is a comma because the sentence has two linked parts in the structure:

• Мало того что ...
• так ещё и ...

The first part is a subordinate clause, and the second part is the main clause that continues the thought. Russian normally separates them with a comma:

• Мало того что я забыла полить кактус, так ещё и оставила его без света.

This punctuation is standard for this construction.

Yes. A simpler version could be:

• Я забыла полить кактус и ещё оставила его без света.

This means roughly the same thing, but it is less expressive.

The original:

• Мало того что я забыла полить кактус, так ещё и оставила его без света.

sounds more emotional and more natural if the speaker is complaining, confessing, or dramatizing their mistake.

So the difference is mostly in tone:

• simpler version = neutral
• original version = stronger, more emphatic, more like not only that, but also...

The overall pattern is fairly fixed:

• Мало того что ..., так ещё и ...

You usually keep those linking words in that order because they work together as a set expression.

Inside each clause, though, Russian word order is flexible. For example, you could change emphasis:

• Мало того что я забыла полить кактус, так ещё и его оставила без света.

This is possible, but it puts extra focus on его.

Still, the original version is the most natural neutral word order:

• Мало того что я забыла полить кактус, так ещё и оставила его без света.

They are similar, but not identical in tone.

• не только ..., но и ... = more neutral, straightforward not only ..., but also ...
• мало того что ..., так ещё и ... = more emotional, often annoyed, upset, or dramatic

So:

• Я не только забыла полить кактус, но и оставила его без света.
  = correct, but more neutral/bookish

• Мало того что я забыла полить кактус, так ещё и оставила его без света.
  = more vivid, more natural for expressing frustration

A good English equivalent is often Not only did I forget to water the cactus, I also left it without light — as if that wasn’t bad enough.