Mi padre reconoce que su peor defecto es ser terco cuando cree que tiene razón.

Not usually in this sentence. Here reconocer que... means to admit, to acknowledge, or to concede that...

So:

• Mi padre reconoce que... = My father admits/acknowledges that...

Spanish reconocer can sometimes mean to recognize in the sense of identifying someone or something, but with que + clause, it often means to admit.

In Spanish, possessive words like mi, tu, su, nuestro normally replace the article.

So you say:

• mi padre = my father
• not el mi padre

This is standard Spanish structure.

It means his worst flaw/defect.

Breakdown:

• su = his/her/your/their
• peor = worse/worst (here: worst)
• defecto = defect, flaw, fault

So su peor defecto means his worst flaw.

In this sentence, su is understood as referring to mi padre.

Because Spanish su is context-dependent.

It can mean:

• his
• her
• your (formal singular)
• their

Here, the subject is mi padre, so the most natural interpretation is his.

If the sentence needed to be extra clear, Spanish could say:

• el peor defecto de mi padre

But su peor defecto is perfectly normal because the context already tells you whose flaw it is.

Peor is the normal comparative/superlative form of malo when talking about something being worse or worst.

So:

• malo = bad
• peor = worse / worst

In this sentence:

• su peor defecto = his worst flaw

Spanish strongly prefers peor here, just like English prefers worst instead of more bad.

Because the sentence is saying what his worst flaw is.

Structure:

• su peor defecto es... = his worst flaw is...

What is that flaw?
ser terco cuando cree que tiene razón

So the infinitive ser is being used as a noun-like idea: being stubborn.

This is very common in Spanish:

• Su problema es llegar tarde. = His problem is arriving late.
• Mi sueño es viajar. = My dream is to travel.

So:

• es ser terco = is being stubborn

Because terco here describes a characteristic tendency or personality trait, not a temporary state.

• ser terco = to be stubborn as a trait
• estar terco can sometimes be used in some contexts to mean someone is acting stubborn at the moment, but that is not the neutral/default choice here

Since the sentence is about his worst flaw, Spanish uses ser, because flaws are understood as more permanent characteristics.

Because it refers to mi padre, who is masculine singular.

Adjectives in Spanish often agree with the person or thing they describe:

• masculine singular: terco
• feminine singular: terca
• masculine plural: tercos
• feminine plural: tercas

Since padre is masculine singular, terco is the correct form.

Because Spanish often omits subject pronouns when they are clear from the verb form or context.

So:

• cree = he thinks / she thinks / you think (formal)
• tiene = he has / she has / you have (formal)

Here, the subject is understood to be mi padre, so Spanish does not need él.

You could say:

• cuando él cree que tiene razón

But it is usually unnecessary, and omitting él sounds more natural unless you want emphasis or contrast.

Because this sentence refers to something habitual or real from the speaker’s point of view, not a future or hypothetical event.

• cuando cree que tiene razón = when he thinks he is right

The indicative (cree) is used because this is describing what he typically does in that situation.

Compare:

• Es terco cuando cree que tiene razón.
  = He is stubborn when he thinks he is right.
  (habitual/real situation)

But Spanish may use the subjunctive after cuando if the event is future or not yet realized:

• Lo hablaré con él cuando crea que tiene razón.
  = I’ll talk to him when he thinks he is right.
  (future reference)

So cree is correct here.

Yes, literally it is has reason, but the expression tener razón means to be right.

It is a fixed Spanish expression.

So:

• Tiene razón. = He/She is right.
• No tienes razón. = You are not right.

Do not translate it word-for-word in normal English.

No. It only means when he thinks he is right.

That part is about his belief, not about whether he is objectively correct.

So the sentence suggests:

• he becomes stubborn
• in situations where he believes he is right

It does not say that he really is right.

A helpful breakdown is:

• Mi padre = subject
• reconoce = main verb
• que su peor defecto es ser terco cuando cree que tiene razón = subordinate clause introduced by que

Inside that clause:

• su peor defecto = subject
• es = verb
• ser terco cuando cree que tiene razón = complement of es

And inside that final part:

• ser terco = being stubborn
• cuando cree que tiene razón = time clause explaining when

So the sentence is basically:

• My father admits
• that his worst flaw is
• being stubborn
• when he thinks he is right

Yes. Depending on context, defecto can be translated as:

• flaw
• fault
• defect
• sometimes weakness in a looser translation

In this sentence, the most natural English choices are probably:

• his worst flaw
• his biggest fault

Defect is accurate, but in English it can sound a little mechanical depending on context.

Because que introduces the clause that tells us what he admits.

• reconoce que... = admits that...

This is very common in Spanish:

• Dice que... = He says that...
• Cree que... = He thinks that...
• Admite que... = He admits that...
• Reconoce que... = He acknowledges/admits that...

So que here works like English that.

Yes, for example:

• Mi padre reconoce que su mayor defecto es ser terco cuando cree tener razón.
• Mi padre admite que su peor defecto es ser terco cuando cree que tiene razón.

A few notes:

• reconoce and admite are both possible, though admite can sound a bit more direct
• peor defecto and mayor defecto are both possible, but they are slightly different in nuance:
  • peor = worst
  • mayor = biggest/main

Also, Spanish sometimes says:

• cuando cree tener razón

instead of

• cuando cree que tiene razón

Both are correct; the original version is very natural and clear.