Перед ужином я вымыл руки.

Because перед normally takes the instrumental case.

So:

• ужин = dinner
• перед ужином = before dinner

This is a very common pattern:

• перед работой = before work
• перед сном = before sleep / before going to bed
• перед уроком = before class

So ужином is simply the instrumental singular form of ужин.

Here перед means before in a time sense.

Russian перед can be used in two main ways:

• spatial: in front of
  • перед домом = in front of the house
• temporal: before
  • перед ужином = before dinner

In this sentence, because ужин is an event/time, the meaning is clearly temporal: before dinner.

This is about aspect, which is very important in Russian.

• мыть / past мыл = imperfective
  Focuses on the process, repetition, or general action of washing.
• вымыть / past вымыл = perfective
  Focuses on the action as completed.

In Перед ужином я вымыл руки, the speaker means they washed their hands and finished doing so before dinner.

Compare:

• Перед ужином я мыл руки.
  This sounds more like I was washing my hands before dinner or I used to wash my hands before dinner, depending on context.
• Перед ужином я вымыл руки.
  I washed my hands before dinner — one completed action.

The prefix вы- helps make the verb perfective and often suggests a result: the washing was carried through to completion.

So:

• мыл = was washing / washed
• вымыл = washed completely / got clean by washing

With руки, вымыл often sounds a bit like washed properly / washed clean.

In everyday speech, both вымыл руки and помыл руки can be used, but вымыл emphasizes the completed result a little more.

Руки here is accusative plural of рука.

The verb вымыть takes a direct object, so hands must be in the accusative case:

• рука = hand
• руки = hands

For inanimate plural nouns, the accusative often looks exactly like the nominative. So although руки looks like the basic plural form, here its grammatical role is accusative plural.

Russian often omits possessive words like my, your, etc. when the owner is obvious from context, especially with body parts and personal belongings.

So:

• Я вымыл руки literally = I washed hands
• natural English meaning = I washed my hands

Russian does this very often:

• Я помыл голову. = I washed my hair / head.
• Он закрыл глаза. = He closed his eyes.
• Она сломала руку. = She broke her arm.

You can say мои руки, but here it would usually sound unnecessary unless you want emphasis or contrast.

Yes, я could be omitted.

Russian often leaves out subject pronouns when the meaning is already clear from the verb form or context.

So both are possible:

• Перед ужином я вымыл руки.
• Перед ужином вымыл руки.

The version with я is perfectly normal. It may sound:

• a bit clearer,
• a bit more neutral in isolation,
• or slightly more emphatic than leaving it out.

In a standalone teaching sentence, including я is very common.

In Russian past tense, the verb agrees with the subject in gender and number.

So:

• я вымыл = I washed (male speaker)
• я вымыла = I washed (female speaker)
• мы вымыли = we washed

So this sentence suggests that the speaker is male. If the speaker were female, it would be:

• Перед ужином я вымыла руки.

Yes. Russian word order is much more flexible than English word order.

These are all possible:

• Перед ужином я вымыл руки.
• Я вымыл руки перед ужином.
• Руки я вымыл перед ужином.

The basic meaning stays the same, but the focus changes slightly.

For example:

• Перед ужином я вымыл руки.
  Emphasizes when it happened.
• Я вымыл руки перед ужином.
  More neutral in many contexts.
• Руки я вымыл перед ужином.
  Emphasizes hands (perhaps contrasting with something else).

The original sentence is very natural and puts before dinner first as the setting.

Not in normal usage.

Technically, перед can mean both in front of and before, but with ужином the normal interpretation is before dinner.

Why?

Because dinner is an event/meal, not a physical object you are standing in front of in any usual sense. So the time meaning is the natural one.

Yes, absolutely.

• помыл руки = washed one’s hands
• вымыл руки = washed one’s hands, often with a bit more sense of completion/result

In many everyday situations, they are very close in meaning.

For a learner, it is enough to know that both can be natural, but вымыл often highlights that the washing was successfully completed.

Not necessarily. It just means before dinner.

How close in time depends on context. In many real situations, it will naturally mean fairly soon before dinner, but the phrase itself does not force an exact time gap.

So it could mean:

• just before dinner,
• earlier before dinner,
• at some point prior to dinner.

Context tells you more precisely.

The normal stress is:

• пере́д у́жином я вы́мыл ру́ки

Useful notes:

• пере́д
• у́жин, so у́жином
• вы́мыл
• ру́ки

Stress matters in Russian, so it is good to learn words together with their stress when possible.