Your 5 Grammar Questions were:
1. When do I use "which" or "that"?
which = non-restrictive clause
that = restrictive clause
Try Grammar Girl for interesting examples
2. When does one use "effect"; use "affect"?
affect is usually a verb
effect is usually a noun
Try Grammarist for interesting examples
3. How many semicolons are appropriate in a standard length paragraph?
Semicolon use is a matter of style connected to the piece of writing. Semicolons are used in lists and to separate phrases.
Generally, the visual of 3 to 5 items separated by semicolons on a list with commas is visually accessible to the reader. Beyond 3-5, the reader may need a bulleted list.
Try About Education for the interesting history and examples of style for semicolons.
4. How does one teach subordinate clauses?
Grammar Monster has the glossary of basic definitions of subordinate clauses and how they are formed. From there use the following teaching strategies to engage students in the making of subordinate clauses.
Try the Sentence Combining Strategy to support your teaching of subordinate clauses.
Try Hot Chalk's example of teaching subordinate clauses through poetry.
5. How do you teach grammar?: (1) How can we teach grammar in a way that isn't boring for students; and, (2) Is it better to teach grammar explicitly or through writing practice?
Grammar can be fun and interesting when it is connected to its purpose, communicating meaning.
Generally, the right/wrong principle of teaching grammar helps students to know the rules but application to editing contexts is the way to achieve active learning and lasting results. Research on traditional grammar teaching is discussed in this article called The Wrong Way to Teach Grammar (The Atlantic, 2014).
Unfortunately, teaching grammar has historically moved between the seesaw of rules/diagramming and using it in context. The solution for teachers lies in activity that games the rules/diagrams for memory and then applies this to the reading/writing connection in student work. Work the seesaw rather than sit on one side. No one can be a better editor or grammar without practice. No one is going to do it or remember it if it does not add purpose and power to your own writing.
Try NCLRC: The Essentials of Language Teaching for some interesting activities.
Teachers need not worry about creating interesting activities for remembering grammar rules these days. Many supportive apps are available.
Try E-Learning Industry and see 11 free iPad apps that you can use as adjunct support in your literary classroom.