There is, and it varies regionally. In UK standard english, the future tense is formed with I shall / you will / he will / we shall / you will / they will. The mixed use of shall / will is difficult, and it is common to use will or shall throughout, the choice varying regionally and (this being the UK) by class. But the point is that DL should accept both.
ihr is irrelevant to the form of the verb because it's not the subject -- the subject is ich.
This is the future tense, so it's ich werde vergessen, with werde conjugated according to ich and with vergessen in the infinitive. (Like "he will forget", which is not "he will forgets".)
It is at first. After a while it gets easier and easier though. Don't sweat the stuff you don't fully understand and just keep moving forward. Like adjective endings used to make me want to bang my head against the wall lol but after just 2 weeks of pain killers... I finally got the concept down and ready for practice. lol
In the official spelling used in German and Austrian schools since 1996, long vowels and diphthongs (ei ai eu äu ie) can only be followed by ß (not ss), while short vowels can only be followed by ss (not ß).
So the spelling to use depends on the pronunciation of the preceding vowel.
A single s can come after a long or a short vowel, though; for example, das has a short vowel and las has a long vowel, but they are not spelled dass (which is a different word) or laß (which does not exist).