Latin does have tips and notes.
At least on the website.
(Don't use apps for learning. They can be good for repeating things you have learned but their lack of access to the website-only tips and notes for many languages are just one reason they're sub-optimal for learning new things.)
I typed Tau si' (grave accent on the i) and was marked correct (a type what you hear exercise). This error may want correcting - the same happening in the "He is and she is" exercise but I didn't check the forum on that)
I've reported it as a "something else went wrong" error.
You're probably misinterpreting the peek.
If you have the sentence "Tá sé" and you hover over "tá", you'll get a hover with "tá sé", then under both of those words "he is", then under the left word only "is", "am", "are". Then if you hover over "sé", you'll get a hover with "tá sé" again, with "he is" under both words, then under the right word only, "he", "it", "six".
So it's telling you that "tá sé" is "he is", and that this phrase is made up of "tá" = "is" (in this case) and "sé" = "he" (in this case).
It's not the case that tá by itself means "he is" and sé by itself also means "he is".
I think it's fairly useful that Duolingo's peek function glosses not only individual words when you peek at them but also combines words from a sentence and tells you how those words, taken together, may have a translation that is not immediately obvious.
(For example, "Tá úll agat" - "Is - apple - at you" may not be enough to enable you to translate it as "you have an apple", but if hovering over the "tá" also tells you that the combination "tá ... agat" can be translated as "you have", that can help.)