You could say mod i too:
(fy) mod i
(dy) fod di/ti
(ei) fod e
(ei) bod hi
(ein) bod ni
(eich) bod chi
(eu) bod nhw
To be honest, some feel that the forms without the the initial pronoun but with the mutation are a bit fake, learner-speak. Because what a lot of native speakers say is:
bod e/o / bo' fe
Can't reply to your comment directly (no Reply button!) but with Welsh you're probably going to have to go with a Welsh for Adults course (one you attend) but spend as much time as possible listening to and reading the real language as well. Duolingo is fine for a quick beginner's intro to the language.
"Meddwl" - Does it mean 'think' in all the ways 'think' can be used in English; as 'believe', 'have the opinion of..', 'have thoughts in general'? The English sentence above can mean a few different things; 'My opinion is that you are ready', 'I believe you are ready.' 'I think you are ready (but am not saying it out aloud). I ask because we have separate verbs for those things in many other languages. To be concrete; is 'credu' and 'meddwl' for instance interchangeable.
"Meddwl" is used to express an opinion e.g "Dw i'n meddwl fod ti'n barod" where "credu"(to believe can also be used), and to actually be thinking in your head e.g "Dw i'n meddwl" is "I'm thinking. It can is also used for "What do you mean?"="Beth wyt ti'n meddwl?" (though "golygu" is used for the meaning of words). I would say that "meddwl" and "credu" are as interchangeable as their English counter parts, but of course sometimes they aren't e.g you don't say "I think in God" and likewise not "Dw i'n meddwl mewn Duw." so "Believe/Credu" is used in this instance." And in my opinion "To think" and "Meddwl" are weaker than "To believe" and "Credu".
That depends on you. It doesn't really matter how long Duolingo takes you - it'll be different for everybody. I find the Western European language courses on here take me a lot quicker to complete than other languages, simply because I'm more familiar with the way those languages work. But that's just me.
The point I was making was that Duolingo is a good way of getting a basic understanding of how Welsh works before moving on to more comprehensive stuff.
It will give you a substantial introduction to Welsh, definitely, but the problem many have is they think doing Duolingo equates to learning the language. Duolingo teaches you the basics of a language through translation and practises your ability to read sentences (without context), understand some grammar and a do a tiny bit of listening. If you think of the skills you need to learn a language like reading, writing, listening, speaking and understanding grammar, you can see how limited Duolingo is.
However, that doesn't mean that you can't learn a lot on Duolingo and that it's not a great free resource - it really is. It's just that it's limited in what it can do.
I'm not saying you do personally, but rather than aiming to "do Duolingo" and getting obsessed with streaks and keeping trees gold, it's better to "do a language" i.e. do a course, attend a class, follow a coursebook, read books, watch videos, listen to stuff, practise with others, ask about grammar and do Duolingo. As EllisVaughan says, it's just one of many resources you combine with others to help you.
So carry on doing Welsh Duolingo at your own pace. Enjoy it, ask questions, use it to learn new stuff, to revise and have fun. But if you feel you want to move on and learn the language in a more complete and rounded way, look into other ways of doing this too and enjoy the challenge. There are plenty of people around here to help. Pob hwyl!
It depends what you mean by substantial. The Welsh course supplies a wide range of grammatical skills and vocab on a wide range of topics. However it will not make you fluent. In fact no one source of language learning will give that to you. That said Duolingo along with "Say something in Welsh" are very good ways to learn Welsh online and I don't believe there are any better options available for free (and very few better paid for ones at that). To continue learning beyond Duolingo I encourage you to try some Welsh media. There are many books suitable for learners and Welsh medium TV is available online on the S4C website if you're in the UK. Beyond this anything from music to twitter can help you develop your language skills. Of course the ideal situation is to actually come to Wales. The national eisteddfod would be a great opportunity to use Welsh and you'll come across people there with all sorts of skill levels. And of course if you have any burning questions the forum resource on Duolingo is pretty invaluable for any language, but especially for Welsh as it's a place you can get reliable infomation on the language at a reasonably quick rate.
My two cents: this is my 101st day of learning Welsh (hurray!) on duolingo and I'm able to read the mynediad part of a book like http://www.gwales.com/bibliographic/?isbn=9781785620621&tsid=3 without too many difficulties.
So duo does give you a very decent foundation I'd say. Diolch i'r cymedrolwyr!