Rozhodli jsme se koupit tento telefon.

The sentence breaks down like this:

• Rozhodli jsme se = we decided
• koupit = to buy
• tento telefon = this phone

So the structure is:

[we decided] + [to buy] + [this phone]

A more literal gloss would be:

Decided we REFL to buy this phone.

That sounds odd in English, but it is normal in Czech.

Because the verb is rozhodnout se, which means to decide.

In Czech, many verbs are used with se as part of their normal dictionary form. So here:

• rozhodnout = to determine / to resolve something
• rozhodnout se = to decide

So in this sentence, se is not optional. It belongs with the verb.

A learner should usually memorize this verb as:

rozhodnout se + infinitive = to decide to do something

For example:

• Rozhodl se odejít. = He decided to leave.
• Rozhodli jsme se koupit auto. = We decided to buy a car.

Rozhodli is the past active participle form used in the Czech past tense.

The past tense in Czech is often built with:

• a past participle
• plus a present form of být (to be)

So:

• rozhodli jsme se = we decided

Here:

• rozhodli shows past tense and plural
• jsme is the auxiliary we are/have used to form the past tense

This is one of the most common Czech past-tense patterns.

Because Czech has special short words called clitics, and jsme is one of them.

Clitics usually like to stand in the second position of the clause. In this sentence:

• Rozhodli is first
• jsme comes right after it

So Rozhodli jsme se is perfectly normal standard Czech.

You may also see other word orders depending on emphasis, for example:

• My jsme se rozhodli koupit tento telefon.

That adds emphasis to we.

But Rozhodli jsme se koupit tento telefon is a very natural neutral sentence.

Because dictionary forms and sentence forms are different.

The dictionary form is the infinitive:

• rozhodnout se = to decide

But in the sentence, the verb is in the past tense:

• rozhodli jsme se = we decided

So you are seeing the verb conjugated, not its dictionary form.

This is similar to English:

• dictionary form: to decide
• sentence form: we decided

Because after rozhodnout se, Czech normally uses an infinitive to say what someone decided to do.

So:

• rozhodnout se koupit = to decide to buy
• rozhodnout se odejít = to decide to leave
• rozhodnout se počkat = to decide to wait

This is very similar to English decide to do something.

So koupit stays in the infinitive because it depends on rozhodli jsme se.

Because Czech has verbal aspect, and here koupit is the perfective verb.

• koupit = to buy, as a completed act
• kupovat = to be buying / to buy repeatedly / to buy in an ongoing sense

In this sentence, the idea is a single completed purchase, so koupit is the natural choice:

• Rozhodli jsme se koupit tento telefon. = We decided to buy this phone.

If you used kupovat, it would suggest a different nuance, such as repeated or ongoing buying, which does not fit as well here.

It is in the accusative, because telefon is the direct object of koupit.

You buy something, so that something goes into the accusative case.

Here:

• tento = this
• telefon = phone

Since telefon is a masculine inanimate noun, its accusative singular looks the same as its nominative singular:

• nominative: tento telefon
• accusative: tento telefon

So the form does not visibly change, but grammatically it is accusative.

Because telefon is a masculine inanimate noun, and in that noun class the accusative singular is usually the same as the nominative singular.

So:

• nominative: telefon
• accusative: telefon

That is why the object still looks like telefon.

Compare that with a masculine animate noun, where the accusative often does change:

• nominative: student
• accusative: studenta

So Czech case is still there, even when the form happens to stay the same.

Both can often mean this, but tento is more explicit or formal.

• tento telefon = this phone
• ten telefon = that phone / that phone there / sometimes simply this phone in everyday speech

In modern spoken Czech, ten is extremely common and often replaces tento in casual conversation.

So a speaker might also say:

• Rozhodli jsme se koupit ten telefon.

That sounds more everyday and conversational.

Using tento is completely correct; it just sounds a bit more precise or formal.

Yes. It tells us something about gender.

In the Czech past tense, plural forms agree with the gender of the group:

• rozhodli jsme se = used for a group containing at least one male, or in mixed-gender/general masculine usage
• rozhodly jsme se = used if the speakers are all female

So this sentence suggests either:

• a mixed group
• a group of males
• or generic masculine wording

If an all-female group were speaking, they would say:

Rozhodly jsme se koupit tento telefon.

Yes. Czech word order is flexible, and changes can affect emphasis more than basic meaning.

For example:

• Rozhodli jsme se koupit tento telefon. = neutral
• Tento telefon jsme se rozhodli koupit. = emphasis on this phone
• My jsme se rozhodli koupit tento telefon. = emphasis on we

However, the placement of short words like jsme and se still follows clitic rules, so not every rearrangement sounds equally natural.

The original sentence is a very normal, neutral way to say it.

No, not in standard Czech if you mean we decided to buy this phone.

The correct verb is rozhodnout se, so se is required:

• Rozhodli jsme se koupit tento telefon.

Without se, the meaning changes or the sentence becomes unnatural in this context.

So it is best to learn the whole pattern:

rozhodnout se + infinitive

A very useful pattern to memorize is:

Rozhodl/a/o/i/y jsem/jsi/jsme/jste se + infinitive

For example:

• Rozhodl jsem se odejít. = I decided to leave.
• Rozhodla se zavolat. = She decided to call.
• Rozhodli jsme se koupit tento telefon. = We decided to buy this phone.

This helps you learn all of these things at once:

• the verb rozhodnout se
• past tense formation
• the role of se
• using an infinitive after decide