Oh, the misuse of that and which. To be honest, when writing a first draft of anything (including this blog post) I over use these words, which many writers do. The confusion of these words comes in the context of adjectival clauses–clauses modifying a noun or a pronoun.
The dogs that ate the food are sick.
This is a restrictive clause expressing how some of “the dogs that ate the food” are sick; not all of them. A restrictive clause restricts the noun.
The dogs, which ate the food, are sick.
This is a nonrestrictive clause that doesn’t restrict the noun. (Note: when using which, commas are mandatory. No exceptions.) Here, all the dogs are sick and all the dogs ate the food.
Both sentences are correct. It depends on the reality of the situation the writer wants to convey. Either all the dogs are sick and ate the food, or only some of them. Powerful writers know the difference, but will still try to find a way to cut out clauses in their writing. Think about Joan Didion or Ernest Hemingway writings. Did they use clauses in their writing? Most likely they cut that and which out, or used it because it was necessary.
When I edit my work or others, I always try to find a way to cut that and which. They aren’t bad words, per se, but overused in general writing. When readers read they don’t want to read that and which multiple times in their writing. Readers want clear, concise writing that flows and has vivid imagery.