Moje maminka říká, že vejce s cibulí jsou dobrá snídaně.

Yes, both moje maminka and má maminka are possible.

• moje maminka is very common and natural in everyday Czech.
• má maminka can sound a bit more literary, emphatic, or contrastive in some contexts.

So in this sentence, moje is just a normal way to say my.

Říká is the 3rd person singular present form of říkat.

So:

• já říkám = I say / I am saying
• ty říkáš = you say
• on/ona říká = he/she says

It is singular because the subject moje maminka is singular.

Because říkat is the imperfective verb, and it is the normal choice for a general present meaning like says.

• říká = says / is saying / keeps saying
• řekne = will say (perfective, usually future in meaning)

So Moje maminka říká... means My mom says..., while Moje maminka řekne... would usually mean My mom will say...

Because že introduces a subordinate clause, and in Czech subordinate clauses are normally separated by a comma.

So the structure is:

• main clause: Moje maminka říká
• subordinate clause: že vejce s cibulí jsou dobrá snídaně

That is why the comma is required.

Yes. Že is the conjunction that.

So:

• Moje maminka říká, že... = My mom says that...

In English, that is often omitted, but in Czech že is usually kept in sentences like this.

This is a very common Czech learner question.

The word vejce can look the same in the nominative singular and nominative plural. So by itself, vejce can mean either egg or eggs, depending on context.

Here, the verb jsou tells you it is plural:

• vejce ... jsou = eggs are
• vejce ... je = egg is

So in this sentence, vejce is understood as eggs.

Because the preposition s meaning with normally takes the instrumental case.

The basic form is:

• cibule = onion

But after s, it becomes:

• s cibulí = with onion

If you wanted plural with onions, that would be:

• s cibulemi

Because dobrá agrees with snídaně, not with vejce.

Here is the structure:

• subject: vejce s cibulí = eggs with onion
• verb: jsou = are
• predicate noun phrase: dobrá snídaně = a good breakfast

Inside dobrá snídaně:

• snídaně is a feminine singular noun
• so the adjective must also be feminine singular: dobrá

So:

• jsou is plural because the subject is plural
• dobrá is feminine singular because it describes snídaně

Because Czech has no articles. There is no direct equivalent of English a/an/the in normal Czech grammar.

So:

• dobrá snídaně can mean a good breakfast
• in another context it could also mean the good breakfast, depending on what is being talked about

Here, the natural English translation is a good breakfast, but Czech does not need a separate word for a.