Crase (from greek krâsis, "mixture", "blend") refers to contraction of two vowels. In portuguese, only two vowels "a" can make up a crase, the first "a" being the preposition a (to, at, in, for) and the second "a" must be the first character of the next word. This phenomenon occurs in the following situations:
- The preposition a meets the defined article a ("the", feminine, regardless number);
- The preposition a meets the pronouns aquele, aquela or aquilo ("that"), regardless genre and number.
Aquele is used to refer masculine words.
Aquela is used to refer feminine words.
Aquilo is like aquelas coisas ("that things").
Sometimes the contraction with the article a occurs even when the next word is implicit/hidden. This situation happens when the hidden word is moda ("mode", "fashion"), maneira ("manner", "way"), a.s.o.
1. Vou à festa de aniversário de Maria.
- I go to Mary's birthday party.
2. Diga àquele homem para parar de beber.
- Tell that man to stop drinking.
3. Saída à francesa.
- Exit without saying farewell. (An idiom; literally, "Exit in the french manner".)
The verb ir (to go, irregular) is constructed with the preposition a. Since the mentioned party (festa) is a specific party (that of Mary), one uses the defined article. Festa is a feminine word so that it receives the article a. This way:
- Vou a a festa → Vou à festa (a+a = à)
Errors in using crase 
Some people write à caminho, à não ser.
But it is not correct because caminho ("way") is a masculine word and não ("no", "not") and ser ("to be") are not nouns.
To be continued.
Author: Daniel Simas Barros
Edited by: Alexandre D. Almeida