Those are Latin nouns; you've got the nominative singular (radius) and the nominative plural (radii).
No; if we were to ADDRESS the "beam; spoke of a wheel; shuttle (in weaving); radius ( = semi-diameter of a circle); beam of sunlight" (these are the definitions of radius given by my little Cassell's dictionary), in the VOCATIVE case, it would be radi . The distinctive ONE "i", versus the two "i"'s that you see in nomin. plur. and genitive sing.
I just wish Duolingo had a course on language terminology! Things like nominative forms and cases don't mean anything to someone like me because I don't know most of those words! Took too long to even fully get how conjugating works, and Im only talking about conjugating in the present tense! I did find an interesting entry level college book about grammar and I'm trying to find these certain needles amongst this whole stack of hay! There's all the more info that Im not looking for too. But if Duolingo had, instead of a tree they could use a flowering plant or a shrub, something smaller than a language tree, that still haf circles to learn thru but we were learning all these grammar terms and how they are used. That would be so cool!
The vocative in -e is for 2nd decl. masc. nouns and adjectives that end in -us. (Rare exceptions: deus, god, uses "deus" as vocative; meus, my, uses "mī" (with long i).) . So, if we have an adjective like scelestus, "wicked," we would call someone "You wicked guy!" to his face by using the vocative sceleste (with the -us, subject form, to -e, "vocative" form, change).
I am starting the course with no previous exposure to Latin, and would tend to agree. Still, it's free and it's interesting to learn some things nonetheless. There is a lot about the grammar that is left unexplained, so the bit that I have learned so far tends to be more through rote memorization of phrases and recognizing words.
Stress falls on the first syllable of each disyllabic word here (the rule is better given as: words of 2 syllables are stressed on the syllable that's "second from the end"). SAL-vē, MAR-ce.
An easy Latin stress-accent rule: a word is NEVER stressed on the final syllable.
(Only the 2nd-from-the-end, or penultimate, or the 3rd-from-the-end, or antepenultimate, can receive the stress, under very clear, cut-and-dried rules:
if the penultimate syllable is LONG, it is stressed. if the penultimate syllable is SHORT, the antepenultimate is stressed.
LONG = it has a long vowel (marked with a macron, or long mark); it consists of a diphthong (ae, au, oe, ei, ui); it consists of a short vowel followed by 2 consonants: such as, the e in puella , the first i in vēnistī , the o in respondeō . )
As explained in other comments on this page, it's the difference between nominative and vocative.
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.