As you state it, it is a matter of graphic art. But you must first think of your “content”. Is it really tabular data? Meaning there is a “natural” ordering in lines and columns and some cells “naturally” call for a summary (like row/column title/caption or bottom sum). If there is no such consistency, then it is only kind of fancy layout (e.g. to improve readability).
For a complete discussion on table/no table dilemma, see the HTML 5 standard in the W3C site.
Intrinsically tabular data is well handled by LO Writer tables. This is the case for a lexicon.
IMHO, syntactic analysis of sentences is not tabular data. It is structural. A good representation is a tree expressing the relationship between hierarchical components. An adequate tool for diagrams is Draw. Components are labelled boxes linked by connectors (so that the edges or arcs remain anchored to the boxes when you move them to find the best layout). If the structure reduces to a one-dimensional representation, you can have a try to the formula editor but I think it is not really fit for that.
If your exercises consist of questions or the like in one column and an empty space in the next, a table could be your choice. But if you propose a sentence with many “holes”, the holes could be in-line frames in a “standard” paragraph.
I would not recommend having all exercises in a single table. There are better way to organise them, e.g. with Heading n paragraphs or a numbered list.
Sorry not to have a definitive answer. Elaborate yourself on my ideas or edit (do not answer) your question to give more details.