Respuesta:
15. How long have you been in this country? (Twelve years)
17. Since when has Mary had her job? (September 2005)
19. How long have you known Mike? (Since we were children)
21. How long have you been a student?
23. How long has your brother been a doctor? (Seven months)
Explicación:
El present perfect se hace uniendo el presente de have con el participio del verbo en cuestión: equivale a la forma "yo he comido" en español. Si aplicamos la estructura de la pregunta en inglés:
Pregunta (what, when, where, how long...) + have + sujeto (you, he, she, I...) + participio del verbo + resto de datos + ?
