Pláštěnka je v batohu.

Because pláštěnka is the subject of the sentence, so it stays in the nominative: pláštěnka.

After the preposition v meaning in, Czech uses the locative case for location, so batoh changes to batohu.

So:

• pláštěnka = nominative singular
• v batohu = in the backpack, with batoh in the locative singular

Je is the 3rd person singular form of být (to be) in the present tense.

So:

• já jsem = I am
• ty jsi = you are
• on/ona/ono je = he/she/it is

In Pláštěnka je v batohu, je simply means is.

Unlike in some other Slavic languages, Czech normally does use the present-tense form of to be in sentences like this.

Because the preposition v requires the locative case when it means in a place.

The noun batoh changes like this:

• nominative: batoh
• locative: batohu

So:

• batoh = backpack
• v batohu = in the backpack

This is a very common pattern in Czech: the preposition affects the form of the noun that follows it.

Czech sometimes uses ve instead of v to make pronunciation easier, especially before awkward consonant clusters.

For example:

• ve škole = in the school
• ve městě = in the city

But batohu starts with a simple b sound, so v batohu is easy to pronounce and is the normal form.

So here, v batohu is correct.

Czech does not have articles like English a/an and the.

That means Pláštěnka je v batohu could be understood as:

• The raincoat is in the backpack
• A raincoat is in a backpack

In real use, context tells you which meaning is intended. Very often, the situation makes it clear whether something is definite or indefinite.

No, Czech word order is fairly flexible.

Pláštěnka je v batohu is a normal, neutral sentence, especially if you are talking about the raincoat and saying where it is.

You could also say:

• V batohu je pláštěnka.

That version shifts the emphasis. It sounds more like you are talking about what is in the backpack.

A useful way to think about it is:

• Pláštěnka je v batohu. = the important new information is where it is
• V batohu je pláštěnka. = the important new information is what is in the backpack

Because v usually means being inside something, while na often means being on a surface or in certain set locations.

A backpack is something you put things inside, so Czech uses v:

• v batohu = in the backpack

Using na batohu would mean on the backpack, which is a different idea.

A rough English-friendly approximation is:

PLAA-shtyen-ka ye v BA-to-hu

A few helpful points:

• á is a long vowel
• š sounds like sh
• ť is a soft t
• ě often makes the preceding consonant softer
• j in Czech sounds like English y, so je sounds like ye

You do not need to pronounce it exactly like English spelling suggests; Czech spelling is quite regular once you learn the letter sounds.