На рубашке намокла одна манжета, потому что я пролила воду.

Рубашке is in the prepositional case because на here means on in a location sense.

• на рубашке = on the shirt
• If there were movement onto the shirt, Russian would usually use the accusative:
  • на рубашку = onto the shirt

So here the idea is that the wet cuff is located on the shirt, not moving onto it.

The verb is намокнуть = to get wet / become wet.

In the past tense, Russian verbs agree with the subject in gender and number. The subject here is манжета, which is feminine singular, so the past tense form is:

• masculine: намок
• feminine: намокла
• neuter: намокло
• plural: намокли

So:

• манжета намокла = the cuff got wet

Because манжета is a feminine noun.

The numeral one changes for gender in Russian:

• один for masculine
• одна for feminine
• одно for neuter

Since манжета is feminine, you need:

• одна манжета

Here одна emphasizes that one cuff got wet, not both.

So the sentence suggests something like:

• one of the cuffs got wet
• possibly with the nuance only one cuff got wet

Without одна, the sentence would still be possible:

• На рубашке намокла манжета.

That would sound more like the cuff / a cuff got wet, with less emphasis on the fact that it was specifically one of the two cuffs.

Russian often does not need to say that as explicitly as English does.

Одна манжета is usually enough for the listener to understand that the shirt has two cuffs and that only one of them got wet.

If you really wanted to be more explicit, you could say:

• одна из манжет = one of the cuffs

But in an ordinary sentence, одна манжета sounds natural.

Russian has no articles like a and the, so it relies on context, word order, and sometimes extra words.

In this sentence, манжета is understood from context as a specific cuff belonging to the shirt.

So depending on context, манжета could be interpreted in English as:

• a cuff
• the cuff

Here, because the shirt is already mentioned, English will often translate it as one cuff or the cuff, even though Russian does not use an article.

Also, одна here is a real numeral meaning one, not an article.

This is a very common question.

• намокла = got wet / became wet
• промокла = got soaked / got wet through

So намокла is the more neutral choice here. It simply says the cuff became wet.

If you say промокла, it often sounds stronger, as if the cuff became thoroughly wet.

So:

• манжета намокла = the cuff got wet
• манжета промокла = the cuff got soaked / wet through

This is about aspect.

• пролила is perfective
• проливала is imperfective

Russian usually uses the perfective here because the speaker means a completed event:

• I spilled water

So:

• я пролила воду = I spilled water

But:

• я проливала воду would sound more like I was spilling water, I used to spill water, or it would need a special context where the action is seen as ongoing or repeated.

For a single completed accident, пролила is the normal choice.

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

So with я in the past tense:

• a male speaker says я пролил
• a female speaker says я пролила

That means this sentence is being said by a female speaker.

If a man were speaking, the sentence would be:

• На рубашке намокла одна манжета, потому что я пролил воду.

Because воду is the accusative singular of вода.

Here воду is the direct object of пролила:

• пролила что? → воду

For feminine nouns ending in -а, the accusative singular usually changes -а to -у:

• вода → воду
• рубашка → рубашку

So:

• я пролила воду = I spilled water

Yes. Russian word order is quite flexible, and changing it changes the focus or emphasis, not the basic meaning.

The original sentence:

• На рубашке намокла одна манжета, потому что я пролила воду.

This sounds natural and sets the scene first: on the shirt.

Other possible orders include:

• Одна манжета на рубашке намокла, потому что я пролила воду.
• Потому что я пролила воду, на рубашке намокла одна манжета.

These are all grammatical, but the emphasis shifts.

The original version is a very natural way to present the information.