If you have to be perfect, but never in speech where perfection is not expected, try
http://www.cysgliad.com/cysill/arlein/.
Put in what you think it is, in Welsh, and not only will it tell you what is wrong, if anything, but why, and give you better suggestions. It’s not a translation tool but an extremely useful grammar checker.
But there will come a time when you will believe @aran and that most people will neither notice nor care about ‘mistakes’ you may be making.
Are you speaking Welsh? Does the other person understand? That’s good enough.