Tu in casual speech
Hi. In Basics 2, from the French course, in the tips and notes, it said: "These other one-syllable words can also elide: je, me, te, se, de, ce, ne, and que. Tu can also be elided in casual speech, but not in writing (including on Duolingo)." Would that mean that you would be able to use "t'as", instead of "tu as"? Just for clarification.
You have it right. "T'as" is a nonstandard conjunction. You will see it online in internet comments, and you will hear it in casual speech (depending on the speaker's accent, it may even sound more like "tcha"), but it is not to be used where good grammar is required. Compare the English word "gonna", a shortening of "going to".
Ya people say t'as / t'es pretty often in casual speech. I didn't know it's improper in writing though, which explains why DL doesn't accept it from me. So then it's the same deal as with "ne" because in spoken French "ne" is often not actually pronounced, but you still need to put it in writing.
I would almost exclusively use the "tu" elision and always drop the "ne" when speaking or writing informally to friends. I might not elide the "tu" if I want to put some emphasis on it.
I think Duolingo is right not to accept these informal variations since you might as well learn the "proper" way, however anybody learning french should be aware of them since they're extremely common in the wild. You're much more likely to hear a random french person say "t'as pas de chance" than "tu n'as pas de chance" for instance.