Mój brat jest wyższy niż mój ojciec.

Because wyższy is the comparative form of wysoki.

• wysoki = tall / high
• wyższy = taller / higher

In this sentence, the speaker is comparing two people, so Polish needs the comparative form, just like English uses taller rather than tall.

It is a common type of Polish adjective comparison, but it is not completely transparent to beginners.

• base adjective: wysoki
• comparative: wyższy

This involves:

• a stem change
• the comparative ending -szy

So you usually have to learn this pair together:

• wysoki → wyższy

This is similar to how English learners must remember pairs like good → better, even when the form is not fully regular.

Jest is the 3rd person singular form of być = to be.

So here:

• mój brat = my brother
• jest = is
• wyższy = taller

Polish normally requires jest in a standard present-tense sentence like this. Unlike in some languages, you do not usually leave out the verb to be in normal modern Polish.

So:

• Mój brat jest wyższy... = correct
• Mój brat wyższy... = not standard as a full sentence

They are both in the nominative here.

Why?

Because the sentence structure is basically:

• My brother is taller than my father

After niż in this kind of comparison, Polish very often keeps the noun in the same form you would expect in a full clause, as if it meant:

• than my father is

That is why you get:

• niż mój ojciec

not:

• niż mojego ojca

in this version.

Yes. That is also correct.

There are two common ways to make this comparison:

• wyższy niż mój ojciec
• wyższy od mojego ojca

The difference is grammatical:

• after niż, you often use the noun in a form matching a full comparison clause, so here mój ojciec
• after od, you use the genitive, so mojego ojca

Both mean the same thing here.

Because this sentence uses niż, not od.

With niż, Polish commonly uses:

• niż mój ojciec

With od, Polish requires the genitive:

• od mojego ojca

So the two patterns are:

• X jest wyższy niż mój ojciec
• X jest wyższy od mojego ojca

A lot of learners mix these, so this is a very normal question.

It is repeated because each noun phrase needs its own possessive word if you want both to mean my.

So:

• mój brat = my brother
• mój ojciec = my father

If you removed the second mój, you would get:

• Mój brat jest wyższy niż ojciec

That can sometimes be understood from context, but it sounds less complete and can feel ambiguous. For a clear beginner sentence, repeating mój is completely natural and correct.

Mój is a possessive word meaning my, and it changes form to match the noun it describes.

Here it appears twice, and both times it agrees with a masculine singular nominative noun:

• mój brat
• mój ojciec

If the noun changed gender or case, mój would change too. For example:

• moja siostra = my sister
• moje dziecko = my child
• mojego ojca = of my father

So mój is not one fixed form in Polish.

Because it agrees with brat, which is masculine singular nominative.

Polish adjectives change to match the noun they describe. Here:

• brat is masculine singular
• so the adjective is wyższy

Compare:

• Mój brat jest wyższy = My brother is taller
• Moja siostra jest wyższa = My sister is taller
• Moje dziecko jest wyższe = My child is taller

So the ending changes according to gender and number.

Polish has no articles.

That means there is no direct equivalent of a or the in ordinary Polish grammar. Whether something is definite or indefinite is usually understood from context.

So:

• mój brat can mean my brother
• mój ojciec can mean my father

You do not need a separate word for the.

In standard Polish, no. For this adjective, the normal comparative is wyższy.

So:

• wysoki → wyższy = correct
• bardziej wysoki = unnatural in normal usage

Polish does sometimes use bardziej + adjective, but mainly:

• with adjectives that do not have a common simple comparative
• or for special stylistic emphasis

For wysoki, learners should use wyższy.

The given word order is the most neutral and natural:

• Mój brat jest wyższy niż mój ojciec.

Polish word order is more flexible than English, but changing it usually changes the emphasis.

For example, you might move words for contrast or style, but the basic beginner pattern is:

• subject + jest
  • comparative adjective + comparison phrase

So this sentence is a very good default model to copy.

These are often tricky for English speakers.

A rough guide:

• wyższy sounds roughly like VYH-zhshih, though English does not match it perfectly
• niż sounds roughly like neezh

Helpful sound notes:

• ż sounds like the s in measure
• sz sounds like sh
• ni before a vowel-like sound is a softened n

Also, Polish stress usually falls on the second-to-last syllable, so:

• WYŻ-szy
• OJ-ciec

A rough pronunciation of the whole sentence is:

• Moy brat yest VYŻ-shy neezh moy OY-chets

That is only approximate, but it can help at the start.

Yes.

• ojciec = father
• tata = dad

So you could say:

• Mój brat jest wyższy niż mój tata.

That sounds a bit more informal and family-like.
Ojciec is more neutral or formal than tata.

Both are correct; the choice depends on tone.