Irregular comparatives
When you want to compare things in English, you usually add "-er" to the end of a word or put "more" in front of it. In Portuguese, the system is much simpler. You usually just place the word mais (more) before the adjective or adverb.
However, a small group of highly common adjectives and adverbs break this rule. Instead of combining with mais, they have special single-word comparative forms. These are known as irregular comparatives. Mastering them is essential because translating "more good" or "more big" literally into Portuguese will immediately mark you as a beginner.
Let us start with the most common irregulars. The adjectives bom (good), mau (bad), and ruim (bad) transform completely when making a comparison.
The comparative of bom is melhor (better). For both mau and ruim, the comparative is pior (worse).
Interestingly, the adverbs bem (well) and mal (badly) share these exact same forms. This means that melhor and pior can act as adjectives describing nouns or as adverbs describing actions.
Size adjectives also have their own special forms. The word grande (big) becomes maior (bigger). The word pequeno (small) becomes menor (smaller).
There is a slight regional difference to keep in mind. Saying mais grande is considered incorrect everywhere. However, when talking about physical size in Portugal, people often use the two-word form mais pequeno instead of menor. In Brazil, menor remains the standard choice for almost all situations.
The most frequent trap for learners is accidentally creating a double comparative. Just as you would not say "more better" in English, you must never say mais melhor or mais pior in Portuguese. The irregular words already contain the meaning of "more".
You might wonder if you can ever say mais bom or mais ruim. You will occasionally hear these combinations, but only in very limited contexts. The most common exception is when you are comparing two different qualities of the exact same person or thing. For example, if you want to say someone is "more good than intelligent", you would use mais bom. For standard comparisons between two different things, always stick to the irregular forms.
When comparing things, remember that the concepts of good, bad, big, and small have special single-word forms. Use melhor, pior, maior, and menor instead of adding mais in front of the base words.