In an official football match, if a player has been substituted, can that player enter in the match again (the trainer must use another substitution)?
In general, no. To quote from the 2015-16 Laws of the Game, page 19:
the substituted player takes no further part in the match
There is however a note on page 18:
The use of return substitutions is only permitted in the lowest levels (grassroots/recreational) of football, subject to the agreement of the member association concerned.