el cuarto grado
You can say "Primer grado es dulce" in Spanish. This phrase captures the idea that first grade is sweet or enjoyable.
You could say "el quinto grado".
It depends on what you mean by "grade" If you are referring to quality, such as "what grade is this cut of meat?" or "I don't care what grade you buy as long as it does the job." The Spanish for "what grade" is "Qué grado?" If you are referring to an academic grade describing how many points you have on an exam or similar, the Spanish for "what grade" would be "Cuál nota?"
Estoy en el tercero grado (I am in the 3rd grade)
cuatro
You can say "Ella era en el sexto grado."
Los cuatro.
Cuádruplex
You say "cuatrocientos libros".
estoy enfadado/enfadada (girl)
estoy en el grado 5
you mean how to say dog in Spanish? well, it is "PERRO" in Spanish. I hope that helps.