V tašce mám malý deštník.

Because v meaning in uses the locative case when it describes a static location.

• dictionary form: taška = bag
• locative singular: tašce

So:

• v tašce = in the bag / in a bag

If you used v taška, it would be grammatically wrong.

Because deštník is a masculine inanimate noun.

The verb mít takes a direct object in the accusative case. But for many masculine inanimate nouns in the singular, the accusative looks exactly like the nominative.

So:

• nominative: deštník
• accusative: deštník

That is why the form does not change here.

Adjectives in Czech must agree with the noun they describe in:

• gender
• number
• case

Here, deštník is:

• masculine inanimate
• singular
• accusative

For masculine inanimate singular, the accusative adjective form is the same as the nominative form, so:

• malý deštník = small umbrella

If the noun were different, the adjective ending would change too.

Czech often leaves out subject pronouns when they are already clear from the verb form.

• mám = I have

The ending -m already tells you the subject is I, so já is not necessary.

You could say:

• Já mám malý deštník.

But that usually adds emphasis, like I have a small umbrella.

Czech word order is much more flexible than English word order.

Starting with V tašce puts the location first. It sounds like:

• In the bag, I have a small umbrella.

A more neutral order is also possible:

• Mám v tašce malý deštník.

Both are grammatical, but the emphasis changes slightly.

• V tašce mám... = focuses first on the bag/location
• Mám v tašce... = more neutral statement

By itself, v tašce just means in a bag / in the bag, depending on context.

Czech does not automatically mean my bag here.

If you want to say in my bag, you would say for example:

• v mé tašce
• v mojí tašce (very common in everyday speech)

So the possessive must be added if it matters.

Because Czech has no articles.

English says:

• a small umbrella
• the bag

Czech usually just says:

• malý deštník
• tašce

Whether something is a, the, or sometimes even my, is often understood from context.

This is a very common and important contrast:

• v tašce = in the bag → location, no movement
• do tašky = into the bag → movement toward the inside

Examples:

• V tašce mám malý deštník. = The umbrella is in the bag.
• Dám deštník do tašky. = I will put the umbrella into the bag.

So:

• v + locative = where something is
• do + genitive = where something goes

• taška is feminine
• deštník is masculine inanimate

This matters because gender affects:

• adjective endings
• case endings
• sometimes pronouns and numerals too

For example:

• v tašce changes because taška is feminine and in the locative
• malý deštník uses the masculine inanimate adjective form

So learning the gender of each noun is very important in Czech.

A few useful pronunciation points:

• š sounds like sh
• c sounds like ts
• Czech stress is usually on the first syllable

So roughly:

• tašce ≈ TAH-sh-tseh
• deštník ≈ DESH-tneek

Also, in normal speech, v before the voiceless t in v tašce is often pronounced more like f, so you may hear something like:

• f tašce

That is normal pronunciation.

Yes. That is grammatical too.

Different word orders are possible:

• V tašce mám malý deštník.
• Mám v tašce malý deštník.
• Mám malý deštník v tašce.

They all express basically the same idea, but the focus is slightly different.

A learner-friendly rule is:

• put the most important or most topical information earlier
• put new or emphasized information later

So the original sentence sounds natural if the speaker wants to begin with the location.