V koupelně vedle zrcadla je ručník a pod ním je mýdlo.

Because v means in here and, when it shows a location, it normally takes the locative case.

• basic form: koupelna = bathroom
• after v for location: v koupelně = in the bathroom

So this is a case change, not a different word.

Because the preposition vedle usually takes the genitive case.

• basic form: zrcadlo = mirror
• genitive singular: zrcadla
• vedle zrcadla = next to / beside the mirror

This is something you mostly have to learn together with the preposition:

• vedle + genitive

Because pod here describes a static location: something is under something else. With that meaning, pod takes the instrumental case.

The pronoun form for he/it in the instrumental singular is ním.

So:

• pod ním = under him / under it

By contrast, něm is a locative form used after other prepositions, for example:

• v něm = in it
• o něm = about him / it

So pod ním is correct because pod + location uses the instrumental.

Grammatically, ním could refer to either:

• ručník = towel
• zrcadlo = mirror

That is because both are singular nouns and both can match ním in this context.

In natural reading, Czech often makes you assume it refers to the nearest suitable noun, so many learners will read it as referring to ručník. But without extra context, the sentence can be ambiguous.

If you wanted to make it completely clear, you could repeat the noun:

• pod ručníkem je mýdlo
• pod zrcadlem je mýdlo

Je is the 3rd person singular of být = to be.

Here it works like is or there is, depending on how you understand the sentence in English.

• je ručník = there is a towel / is a towel
• je mýdlo = there is soap / is soap

It appears twice because the sentence contains two separate statements joined by a = and:

1. V koupelně vedle zrcadla je ručník
2. pod ním je mýdlo

So Czech repeats je just as English would usually repeat is or there is.

Both are possible, but they sound a little different.

Czech word order is more flexible than English, and it often depends on information structure:

• what is already known
• what is new or important

V koupelně vedle zrcadla je ručník starts with the place, so it sounds like someone is describing a scene: first the setting, then what is there.

Ručník je v koupelně vedle zrcadla puts more focus on the towel itself.

So the original sentence is very natural if you are visually describing where things are.

Because Czech has no articles.

English distinguishes:

• a towel
• the towel

Czech usually does not mark that difference with a separate word. Instead, the meaning comes from:

• context
• word order
• emphasis
• sometimes pronouns such as ten, ta, to

So ručník can mean a towel or the towel, depending on context.

The same is true for mýdlo.

Here is the full breakdown:

• v koupelně

  • v
    • locative
  • koupelně is locative singular of koupelna

• vedle zrcadla

  • vedle
    • genitive
  • zrcadla is genitive singular of zrcadlo

• je ručník

  • ručník is nominative singular

• pod ním

  • pod
    • instrumental for location
  • ním is instrumental singular pronoun

• je mýdlo

  • mýdlo is nominative singular

This is a very useful sentence because it shows several common case patterns with prepositions.

Yes, it matters.

• ručník = masculine inanimate
• zrcadlo = neuter
• mýdlo = neuter

Gender matters because it affects:

• adjective endings
• pronouns
• some past tense forms
• some case endings

For example, the pronoun ním can refer to a singular masculine or neuter noun, which is one reason why pod ním can be ambiguous here.

Yes. Czech often changes case depending on whether you mean:

• location = where something is
• movement/direction = where something goes

In this sentence, everything is about location:

• v koupelně = in the bathroom
• pod ním = under it

But compare:

• Je v koupelně. = It is in the bathroom.
• Jde do koupelny. = He/she is going into the bathroom.

And with pod:

• Je pod stolem. = It is under the table.
  
  • location → instrumental
• Dám to pod stůl. = I will put it under the table.
  
  • direction → accusative

So case choice in Czech often depends on whether something is stationary or moving.