Learn Portuguese (Portugal): Seja sólido seja mole, custa-nos a trincar o pão. - Whether it is solid or soft, we find it hard to bite the bread.

In the structure custar a plus an infinitive, the verb custar does not agree with the person finding it hard. Instead, the action itself, which is trincar o pão, acts as the grammatical subject. Literally, the sentence is saying that biting the bread is hard for us. The person experiencing the difficulty is expressed as the indirect object pronoun nos.

Is the preposition a mandatory in the phrase custa-nos a trincar?

In traditional European Portuguese, when custar means to be difficult, it pairs with the preposition a before the infinitive. This is the specific verb valency anomaly this lesson covers. However, in modern everyday Portuguese, you will also hear people drop the a and just say custa-nos trincar. Both are understood, but keeping the a is very typical of the European variety.

Why do we use the subjunctive form seja twice at the beginning?

Repeating the present subjunctive like this is a common Portuguese structure used to mean whether X or Y. Literally, it translates to be it solid, be it soft. You can apply this repeating pattern to almost any contrasting pair, such as seja hoje seja amanhã for whether it is today or tomorrow.

I learned the verb morder for to bite. How is trincar different in this context?