Translation:I gave a letter to my girlfriend.
A better translation here would be: "I gave her a letter."
You can infer that the かのじょ here refers to the speaker's girlfriend but it's not definitely indicated by the sentence. So it could be, "I gave my girlfriend a letter."
But "I gave to my girlfriend a letter" is really awkward English phrasing.
No, it REALLY doesn't. Duo needs to up their Japanese game.
First, "watashi" is not a verb, and cannot be conjugated, but "mashita is obviously a "masu" suffix. Dropping the suffix, you get the stem "watashi", not the noun.
As the "masu" form replaces the ending "u" with "i" or "e", mostly without changing the consonant, you get "watasu" as the verb.
There's a really short list of rules for the possible stem changes based on the ending syllable. Why's it not included on Duo?
I would like to see more kanji generally speaking, but I see your point here. However, I think Erika's talking about confusion in the word-bank questions, where "watashi" and "mashita" might be separated, with other stems mixed in. I don't know what the teaching theory is, and maybe they're trying to intentionally be confusing, since you sometimes have to deal with different kinds of ambiguity in Japanese (along the lines of dajare), and you just have to know that particular word.
As for including a list of rules for stem changes, Duolingo is explicit about this, intentionally being light on rules instruction, making any sort of formal grammar instruction optional. They're going for a more examples- and immersion-based approach. There are certainly pros and cons to it.
That said, there are tips with points on grammar for some sections by selecting the lightbulb, which may have some information on these conjugation rules, but again, they are designed to be optional.
Is there a relation between watashi and watasu? Together with the sound, the meanings are very similar, too, if we think to "watashi" as a submissive form of courtesy for "I". I know that the kanji are different, but that could have happened because of the different time the two kanji were imported from China. Does anyone know something about this?
The pronoun watashi (私) is a short form of watakushi. I don't know if watakushi (私) is etymologically related to watasu (渡す), but at least the Kojien dictionary doesn't suggest that.
According to Kojien, watakushi originally refers to one's own matters, which is exactly what 私 means in Chinese ("private"). As to watasu, one of its meanings is to ferry someone across a river (cf. wataru "to cross a river"), and 渡 is the natural choice of Chinese character for this sense. So I don't think the time when these characters entered Japanese matters here.
I really fail to see the difference to give. Maybe it's because my English is not very good.
Isn't Hand in something more official like giving an exam back to the teacher.
I mean is it common to say I handed over something to my gf.
If the usage is different in Japanese, could someone please explain it.
I think handing something to someone--or handing it in, or handing it over--implies a physical exchange. An object went from one person's hand to another, like a test paperto a teacher. Give seems more general--you can physically give someone a gift, or you can give them a kiss (not hand-related), or you can give them the time of day or a piece of your mind (again, nothing's changing hands). I don't think of handing in as being more formal, although I suppose it's usually used in a classroom setting. "Hand it over" generally makes me think of someone with something that isn't rightfully theirs or someone who is in trouble. A policeman might say "Hand it over!" to someone with stolen goods or a gun or a piece of evidence. Or a mom might say it to her kid who is hiding something he's not supposed to have behind his back.