Jejich vztah je dobrý.

Yes. Jejich means their.

It shows that something belongs to or is associated with them:

• jejich vztah = their relationship

Unlike English, there is no apostrophe involved. It is simply the normal Czech possessive form.

Also, jejich refers to a plural possessor, regardless of gender:

• their = jejich
• whether they are men, women, or a mixed group

Because the subject is vztah, and vztah is singular.

• vztah = relationship
• one relationship = singular
• so the verb must also be singular: je = is

A native English speaker may notice jejich looks plural because it means their, but that does not control the verb. The verb agrees with vztah, not with the possessor.

So:

• Jejich vztah je dobrý. = Their relationship is good.
• If the subject were plural, then you would use jsou.

Vztah is a masculine noun.

Because Czech adjectives agree with the noun they describe, dobrý has the masculine singular form.

So:

• masculine singular: dobrý
• feminine singular: dobrá
• neuter singular: dobré
• plural: dobré or dobří, depending on the noun type

Here:

• vztah = masculine singular
• so dobrý is the correct form

This is one of the biggest differences from English, where good never changes form.

Vztah is in the nominative case because it is the subject of the sentence.

In Jejich vztah je dobrý:

• jejich vztah = the subject
• je = the verb
• dobrý = a predicate adjective describing the subject

So the basic grammar is:

• subject in nominative
• verb
• adjective agreeing with the subject

In this possessive use, jejich stays jejich.

That is helpful for learners because many Czech words change endings, but jejich is much more stable in this kind of phrase:

• jejich vztah
• vidím jejich vztah
• bez jejich vztahu

The noun changes when needed, but jejich itself remains the same.

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

So Jejich vztah je dobrý can naturally mean:

• Their relationship is good

Czech relies on context, word order, and general meaning instead of articles.

That is normal and not something missing from the sentence.

Yes, Czech word order is more flexible than English, although Jejich vztah je dobrý is the most neutral order.

This version is straightforward:

• Jejich vztah = topic / subject
• je dobrý = what is being said about it

Other word orders are possible, but they usually sound marked or emphasize something:

• Dobrý je jejich vztah. = more emphasis on good
• Je dobrý jejich vztah. = possible in special contexts, but less neutral

For learners, the safest default is:

• Jejich vztah je dobrý.

A simple learner-friendly approximation is:

YEH-yikh FSTAHKH yeh DOH-bree

A few useful notes:

• jejich begins with ye-
• ch is the Czech sound like the ch in German Bach or Scottish loch
• vztah begins with a difficult consonant cluster; many learners need practice with it
• dobrý has a long ý, so the last syllable is held a little longer

Stress in Czech normally falls on the first syllable of each word:

• JE-jich
• VZTAH
• DO-brý

No, not in a normal present-tense Czech sentence like this.

Czech normally keeps the present-tense form of být:

• je = is

So:

• Jejich vztah je dobrý. = correct
• Jejich vztah dobrý. = not standard Czech

For English speakers, this is actually convenient, because Czech behaves more like English here than some other Slavic languages do.

Because svůj is a reflexive possessive, and it must refer back to the subject of the same clause.

In this sentence, the subject is:

• vztah

So svůj would not make sense here as their.

You use jejich because the possessor is they, not the grammatical subject of the clause.

So:

• jejich vztah = their relationship is correct

This is a very common point of confusion for learners, and the key rule is:

• use svůj only when it refers to the subject of the same clause

No. In this sentence, the adjective must agree with vztah, and vztah is masculine singular.

So the correct form is:

• dobrý

These would be wrong here:

• dobrá = feminine singular
• dobré = neuter singular or some plural uses

So the full agreement is:

• jejich vztah = masculine singular
• je dobrý = masculine singular adjective form