Takového zloděje jsme neviděli.

Because zloděje is the direct object of neviděli, so it has to be in the accusative case.

For zloděj (a masculine animate noun), the forms are:

• takový zloděj = nominative singular
• takového zloděje = accusative singular

The word takový has to agree with zloděj in case, number, and gender, so it changes to takového.

So:

• Takový zloděj... = Such a thief... as the subject
• Viděli jsme takového zloděje. = We saw such a thief.

It is accusative singular.

In this sentence, zloděje is the thing being seen, so it is the direct object of the verb vidět / nevidět.

Because zloděj is masculine animate, its accusative singular looks like this:

• nominative: zloděj
• accusative: zloděje

That is why you get takového zloděje.

Here takového means something like such a, that kind of, or a thief like that.

It often adds emphasis. So the sentence does not just mean we didn’t see a thief. It means more like:

• We haven’t seen such a thief before.
• We had never seen a thief like that.

So takového points to the unusual or striking nature of the thief.

Because Czech usually drops subject pronouns when they are not needed.

The idea of we is already contained in jsme, which is the 1st person plural past-tense auxiliary.

So:

• Takového zloděje jsme neviděli. = We didn’t see such a thief.

You could add my:

• My jsme takového zloděje neviděli.

But that sounds more emphatic, like we didn’t, perhaps contrasting with someone else.

Czech past tense is built from:

• a past participle: viděli
• a form of být as auxiliary: jsme

So literally the structure is something like we are seen historically, but in modern Czech it simply means we saw / we have seen.

The important thing is that short forms like jsem, jsi, jsme, jste are clitics. They usually go in the second position of the sentence.

That is why:

• Takového zloděje jsme neviděli.

After the first phrase Takového zloděje, the clitic jsme comes next.

Yes, absolutely.

Both are grammatical:

• Takového zloděje jsme neviděli.
• Neviděli jsme takového zloděje.

The difference is mainly word order and emphasis.

• Takového zloděje jsme neviděli. puts emphasis on such a thief
• Neviděli jsme takového zloděje. is more neutral and starts with the verb

Czech word order is flexible, and speakers use it to highlight different parts of the sentence.

Because the past participle agrees with the subject in number and, in the plural, also partly in gender.

• neviděli is used for a group containing at least one male person, or a mixed group
• neviděly is used for an all-female group or for non-masculine subjects

So this sentence suggests that we is either:

• a mixed group, or
• a group of males

If the speakers were all female, you would say:

• Takového zloděje jsme neviděly.

Because vidět is the imperfective verb and is natural for talking about general experience or the state of having seen something.

This sentence is about experience: we haven’t seen / had not seen such a thief.

If you used uvidět, the meaning would shift more toward catch sight of, manage to see, or a single completed moment of seeing.

Compare:

• Takového zloděje jsme neviděli. = We hadn’t seen such a thief / We have never seen such a thief.
• Takového zloděje jsme neuviděli. = We did not catch sight of such a thief.

For this sentence, neviděli is the natural choice.

Usually, in modern Czech, no—not in a sentence like this.

So after neviděli, you normally keep the object in the accusative:

• Takového zloděje jsme neviděli.

Older Czech and some formal or literary styles can use genitive after negation, but that is not what learners should normally use here.

So the safe modern rule is:

• with nevidět, use the normal direct-object accusative unless you have a specific reason not to

It can depend on context.

The plain past form jsme neviděli can often correspond to English:

• we didn’t see
• we haven’t seen
• we had not seen

In a sentence like this, especially with takového, it often has the broader sense of experience:

• We have never seen such a thief.
• We had never seen a thief like that.

So even though the Czech form is simple past, the real English translation may need present perfect or past perfect depending on the situation.