Вона витерла пляму за допомогою води.

Because витерла is the past tense of a perfective verb: витерти.

• витерти = to wipe off / wipe away / wipe clean, with the result completed
• витирати = to be wiping / to wipe repeatedly / to wipe in general

So:

• Вона витерла пляму = She wiped the stain off / She removed the stain
• Вона витирала пляму = She was wiping the stain or She used to wipe the stain

In this sentence, the action is presented as finished, so витерла is the natural choice.

That ending shows past tense feminine singular.

In Ukrainian past tense, verbs agree with the subject in gender and number:

• він витер = he wiped
• вона витерла = she wiped
• воно витерло = it wiped
• вони витерли = they wiped

Since вона means she, the verb must be витерла.

Because it is the direct object of the verb, so it goes into the accusative case.

Dictionary form:

• пляма = stain

Accusative singular:

• пляму

So:

• витерла що? = wiped what?
• пляму

This is a very common pattern with feminine nouns ending in -а:

• книга → книгу
• ручка → ручку
• пляма → пляму

За допомогою means with the help of, by means of, or more simply using.

So:

• за допомогою води = with the help of water / using water

This is a fixed expression you will see often in more formal or neutral Ukrainian.

Examples:

• за допомогою комп’ютера = using a computer
• за допомогою ножа = using a knife

Because за допомогою requires the genitive case.

The noun вода changes like this:

• nominative: вода
• genitive: води

So:

• за допомогою води = using water

This is something you mostly just learn together with the expression:

• за допомогою + genitive

Yes, often you can say:

• Вона витерла пляму водою.

Here водою is the instrumental case, and the sentence means roughly She wiped the stain with water.

Both versions are understandable, but there is a slight stylistic difference:

• за допомогою води = a bit more formal, more like by means of water
• водою = simpler, more natural in everyday speech in many contexts

A learner should know both patterns:

• за допомогою + genitive
• instrumental for the means/tool

It can often be omitted.

Ukrainian frequently drops subject pronouns when the verb already makes the subject clear:

• Витерла пляму за допомогою води.

Because витерла is feminine singular past tense, it already suggests she.

You keep вона when:

• you want emphasis
• you are contrasting her with someone else
• you want to make the subject especially clear

For example:

• Вона витерла пляму, а він ні. = She wiped the stain off, but he didn’t.

No, Ukrainian word order is fairly flexible.

The neutral order here is:

• Вона витерла пляму за допомогою води.

But other orders are possible depending on emphasis:

• За допомогою води вона витерла пляму.
  Emphasis on using water
• Пляму вона витерла за допомогою води.
  Emphasis on the stain
• Витерла вона пляму за допомогою води.
  More marked, often contextual or stylistic

So the basic meaning stays the same, but the focus can shift.

In this sentence, витерти means to wipe something off / away / clean so that it is removed.

That is slightly different from just терти, which means to rub.

Compare:

• терти = to rub
• витирати / витерти = to wipe, wipe away, wipe off

So витерла пляму suggests not just rubbing the stain, but successfully removing it by wiping.

Often yes, but the exact English word depends on context.

Пляма can mean:

• stain
• spot
• mark
• blotch

Examples:

• пляма від кави = coffee stain
• темна пляма = dark spot
• пляма на сорочці = stain/spot on a shirt

So in your sentence, пляму is most naturally the stain.

The phrase за допомогою води tells you the means used to perform the action.

A useful way to analyze the sentence is:

• Вона = subject
• витерла = verb
• пляму = direct object
• за допомогою води = means/method (using water)

So if you ask:

• What did she wipe off? → пляму
• How / by means of what? → за допомогою води

This kind of breakdown is very helpful when reading Ukrainian sentences.