"I like Monday."
Translation:Dw i'n hoffi dydd Llun.
If anyone's still unsure, you'll do fine with dw i - it's what the course (and the Welsh curriculum) teaches. It's pretty simple and understood by everyone.
Some of the other answers were added after being reported as correct by users, and these make their way into multiple choice questions. Some of these are archaic/dialectical/context-dependent. You'll recognize the other forms with time (clue in the pronoun i, ti, dych etc.) but there's no need to worry too much if you want to keep things simple.
There are slight riffs on it for questions and negative statements, but nothing drastic. I gave a link (and a fuller explanation!) in my other comment on this discussion to a table of different forms, so you can check that out if you like.
You can't go wrong with dw i, so maybe stick with that for simplicity. It's what nearly everyone actually says. The others are less common.
(ry)dw i "I am"
(r)wyt ti "you are". This is singular informal.
mae e/hi "he/she is". (Northern: mae o/hi)
(ry)dyn ni "we are" (Northern: (ry)dan ni)
(ry)dych chi "you are" (Northern: (ry)dach chi)
maen nhw "they are"
There are (very) slightly different forms for questions and negative statements. But the table in this link should show them in a nice, clear way. There's one typo: ddyn ni ddim should be dyn ni ddim: https://en.m.wikibooks.org/wiki/Welsh/Sylfaen/Gwers_1#Y_Presennol
Wiktionary says rydw "is a normalization found primarily in pedagogical material; it is rarely encountered either in colloquial speech or in literature" (https://en.wiktionary.org/wiki/rydw). Rydw, rydyn and rydych used to be widely taught, but not so much now. The r- forms tend to be used in affirmative statements, but other than rwyt, not in all parts of Wales. They don't appear in questions, and rarely in negative statements. You don't have to worry too much about them, and you'll come to recognise them with time on the rare occasions this course uses them.
If you're interested in getting deeper into the other forms, this forum post may be helpful in explaining why there are so many different forms of 'I am', which might help pin down some patterns. It seems prefixes kept getting added and worn away again at different times by different dialects: http://www.fluentin3months.com/forum/specific-language-questions/cymraeg-differences-between-rydw-i-dw-i-dwi-etc/
Yes, they can, but afaik not completely alone, but as part of a segment: e.g. Bore Dydd Llun (Monday morning) is usually said Bore Llun... Same for Noswaith Gwerner, etc. And while Irish 'Dé' is not always necessary, it's used in most situations. Maybe in a later lesson we find out the informal or regional variants do, in fact, leave dydd off.
That assimilation is quite common, it usually happens when a word ends in vowel and the next starts with the same vowel. The evolution goes something like French words "de" and "amour": de a'mu:r > da 'amu:r > da:'mu:r > da'mu:r (written d'amour).
If you listen to how those three words sound, you will notice that the "w" makes the "i" a bit closed (lips a bit like to say an "u"), and closer to sounding like "y" (let's mark it as "iy"), so, it's almost like it sounds "Dw y yn" On the other hand, the "n" makes the "y" sound less "y" (as part of the air move to the nose), so it's closer to a shorter and a bit more open "y", let's call it "¡". Thus, you'd say something like "Dw iy ¡n hoffi..." Now, so many vowels in a round! The air flows no stop until you hit the 'n', so this is like saying "dwin", hence "dw i'n" We need the written "dw i" to know the verb: tense, person, but we have to mark the existing "yn".
Hope this clears things up... :-)
I'm not completely certain, but I think the day name is "Dydd Llun". Thus "Bore Dydd Llun". Apparently, when it works as a kind of adjective (for an adverb, for example), like in Monday morning, the Dydd part can be rid off, thus Bore Llun. However, in Dw i'n hoffi Dydd Llun, it's working as a direct object (accusative) and thus the whole name applies. Anyway, remember that languages are not necessarily logical, even within themselves... If they have managed to keep the Latin Dies Lunae almost literally, it's because they do like the Dydd in that place... :-) For reference, all Romance languages (possibly including Romanian, I dunno) have ditched or integrated into the name the Dies part: lunes, martes, miércoles... dilluns, dimars, mercredi... YMMV :-)
Romance languages are spanish, french, portuguese, etc---actually a bunch of them---, yes. Most of them, if not all, have ditched of assimilated the "dies" part into the day name [except portuguese, which changed them for being too pagan, so it's second day--segunda feira--, third, etc until sábado and domingo, from sabbat and dominus dies]. Welsh is not a romance language, far from it, as it's from the celtic family of languages... however, it is related to latin, as both come from a common ancestor, a italian-celtic branch of the indoeuropean languages... apparently celts on the north of italy, down to the Po river, and Gauls as well in france and belgium, would speak a celtic language which was quite similar to latin... however, Wales was under Roman domain for several centuries, and the day names --- being related to religion in Rome and coming from christianizing efforts --- are clearly the latin ones... it's not surprising, actually... peace!