Můj telefon má novou baterii.

Because telefon is a masculine singular noun, and můj has to agree with the noun it describes.

In Czech, possessive words like můj change form depending on the gender, number, and case of the noun after them.

Here:

• telefon = masculine singular nominative
• so the correct form is můj telefon

Compare:

• můj telefon = my phone
• moje baterie = my battery
• moje auto = my car

So the form depends on the thing owned, not on the owner.

Má is the 3rd person singular present tense of the verb mít, which means to have.

A few present-tense forms are:

• já mám = I have
• ty máš = you have
• on / ona / ono má = he / she / it has

Since telefon is a singular subject, Czech uses má.

Because novou baterii is the direct object of the verb má.

In Czech, the direct object of an affirmative transitive verb usually goes into the accusative case.

So:

• nominative: nová baterie
• accusative: novou baterii

Both words change:

• nová → novou
• baterie → baterii

English does not usually show this kind of change, but Czech does.

Because baterie is one of the feminine nouns whose ending changes to -ii in certain cases, including the accusative singular.

So:

• nominative: baterie
• accusative: baterii

This is normal for many nouns ending in -ie, for example:

• teorie → teorii
• stanice → stanici
  (similar idea, though not exactly the same spelling pattern)

So the double i is not a typo. It is the correct case ending.

You usually learn a Czech noun together with its gender. In dictionaries, baterie will be marked as feminine.

Its form also fits a common feminine pattern:

• many Czech nouns ending in -e or -ie are feminine

And the adjective confirms it:

• novou is a feminine singular accusative form

So in this sentence, both the noun and the adjective show that baterie is feminine.

Because Czech has no articles.

Unlike English, Czech does not have separate words for a, an, or the. Whether something is definite or indefinite is understood from context.

So novou baterii can correspond to:

• a new battery
• sometimes the new battery

In a simple sentence like this, English will often use a new battery, but Czech itself does not mark that distinction with an article.

Czech word order is more flexible than English word order because the endings show what each word is doing.

Můj telefon má novou baterii is the most neutral order.

But other orders are possible, for example:

• Novou baterii má můj telefon.
• Můj telefon novou baterii má.

These alternatives sound more emphatic or contrastive. The basic meaning stays similar, but the focus changes.

So the given sentence is the normal, neutral way to say it.

Because svůj is a reflexive possessive. It refers back to the subject of the clause.

In this sentence, the subject is telefon, not I.

So:

• můj telefon = the phone belongs to me
• svůj would refer back to telefon

That means svou baterii would mean something like its own battery. That can make sense in some contexts, but it is not the normal way to say my phone here.

So:

• Můj telefon is correct for my phone
• svůj is used when the possessor is the subject, for example:
  • Mám svůj telefon. = I have my phone.