tdl mode gives emacs some expectations about the syntax of tdl files. Based on those expectations, emacs will automatically align any line if you put the cursor on it and type tab.
If your syntax is correct, lines beginning with a feature name will align so that features of the same type (= features at the same depth within the feature structure) will be in the same column. Lines beginning with [ will line up so that the [ is in the same column it would be in if you hadn't put a newline before it.
If your syntax is incorrect, emacs will do its best to align things based on the syntax you have actually coded. This usually means that the feature or bracket will align somewhere that you can tell is wrong.
So, step by step: