Aquí or Aca means here or the location actual of a person or something. It can also be used to mean now as in the expression "aqui comienza el programa", "now the program starts".
Examples: "Venga por aca" or "ven aca" a commonly used expression which means come here. It might be a parent talking to a child to come from another room to where the parent is located. You could also say in Spanish, "Venga por aqui" to mean the same thing.
These are also used to mean "here" and "there", "aca y alla", or "desde entonces aca" meaning "since then", also "mas aca" meaning "closer" or "nearer", in addition to "muy aca" to mean " very near" or "very close".
Allí (there, over there) a demonstrative adverb meaning relatively near to you or the point of reference. For instance, if you are in the house and a family member asks you to move over, so you go across the room to another location and say to them, "Voy por alli" or "I’m going over there".
Examples: "Estoy alli" You arrive at your friend’s apartment building, You call him on your cell phone and he asks where you are. You answer, "I am here".
"Alli"" can also be used to mean "then", or "in there" as in "alli dentro", or "that way" as in "por alli"
Alla is a demonstrative for "there" which is farther away than with alli
Examples: "El esta alla en su oficina" or "He is there in his office". The man is in the city but not right in the same general location or vicinity as the speaker. It can also used to mean "back then" as in "alli en verano", "back in the summer", or "mas alla", as "farther on", or "mas alla de",as in "farther on than", or "en el mas alla" which means "beyond"
Aculla is used to mean "over there" or "here" inside the relative location or city and farther away than alla mentioned above. Example: "La sede de la Universidad donde el estudia esta acuya".This expression is occasionally heard in Venezuela, Colombia, Ecuador and Peru through Argentina and Uruguay but is much less common in other Spanish-speaking countries where "alla" is more commonly used.
Allende or Allende de los mares refers to far beyond a location or a great distance away, another country, or overseas (en Bogota) The university branch where he studies is there (in the city of Bogota).This is another South American regionally accepted term which has far less frequency of use in Europe, the Caribbean and Central American Spanish-speaking countries.
Example: "Mi hermano esta allende de los mares". "My brother is overseas." "Where?" "In Italy." "¿Donde?" " En Italia".