Two things here:
- "Did see" is a past-tense verb construction.
- "Seen" is not a past-tense verb, but a past participle. The past-tense verb would be "saw".
"Have seen" is the present perfect tense, which is quite different from the simple past, and it has a separate translation:
- ¿Has visto mi diccionario? - Have you seen my dictionary?
miraste or viste. Is one looking at or into the dictionary. Even with my own family and peers there is confusion with the natural ambiguity of words. Especially when we try to say something with few words. So these distinctions Duo users discuss are very interesting and significant. Thanks
Italiano is an adjective if you use it to describe a noun as being of Italian origin. But italiano is also a noun, referring to the Italian language. The point of this sentence is that it's a dictionary that busies itself with the language, not a dictionary that's from Italy or written in Italian.
Compare with this:
- mi maestro de español - the teacher that teaches me Spanish; referring to the language
- mi maestro español - my teacher who is from Spain; referring to his origin