• The article precedes only nouns or words with noun value. For this reason, the crasis will not come in front of verbs, nor personal pronouns (subject).
However, both the preposition “a” and the feminine article “a” will come in front of feminine nouns, since masculine nouns do not admit feminine article.
Watch:
I will not go à drugstore. I will go to the supermarket.
The verb “to go” requires a preposition, see: I will not. At where? THE somewhere. Which? THE drugstore. who goes, goes The somewhere. In answer "The which place?" we have the article “a”. Therefore, the preposition "a" plus the feminine article "a", which accompanies the noun in the answer (The pharmacy), form the backlash.
Now, watch:
I don't want to read The cover of this book.
The verb "to read" or the verb phrase "I want to read" does not require preposition, therefore, the term “a” that is in the sentence above is a feminine article.
I declared The he that I'm innocent.
In the sentence above, the personal pronoun "he" does not admit
• Preposition "a" and demonstrative pronouns
The demonstrative pronouns in which the crasis can occur are: that, that, those, those, that, a(s). For this, the regent term must require preposition. For example:
I attended that one horrible TV show.
That one call it an educational program?
I mean that one studious student.
Sabrina Vilarinho
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/crasepreposicao-artigo-feminino-a.htm