See the course notes for explanations and examples.
- mewn - in somewhere/thing general or unspecified - mewn car; mewn tŷ (in a car; in a house)
- yn/yng/ym - in somewhere/thing specific - ym Mangor; yn y car; yng nghar Dewi (in Bangor; in the car; in Dewi's car)
To see how to find the course notes - at the top of this page click on 'Topic: Welsh', then on 'Popular', and then look for the sticky discussion 'Course hints and tips'.
I agree that the best thing is to read the notes (which I can't find on the app but only on the website - or follow the link above), but the answer the two issues in the questions: firstly the r-forms are the posh forms used in positive statements only, so you may be confused by, for example, rydw and dw appearing to mean the same thing. Secondly, Welsh (almost) always put the verb before the subject, like they do in all Celtic languages, but like we do only in questions in English (am I).
The rwyt in rwyt ti is the 'are' in 'you are'.
See the conjugation of the present tense of bod (being, to be) in the course notes (https://forum.duolingo.com/comment/17638579) for the section 'Present 3' - https://www.duolingo.com/skill/cy/Present-3/tips-and-notes
Yes and no...
The basic form of a verb is its verb-noun, for example gweithio, mynd, bwyta. There are conjugated forns of nearly all verbs, but some of those forms are little used except in more formal registers of the language. There is also a system of using auxiiary verbs with verb-nouns to create compound verbs - mainly bod (being, to be) and the simple future and simple past tenses using gwneud (doing, to do).
We start with using bod with verb-nouns to create the present tense. This is explained in the course notes.
Later in the course we cover other tenses using bod and gwneud and some colloquial uses of conjugated verbs in other tenses.