Knowing when to use "I work" (habit) vs. "I am working" (right now).
Using who, which, and that to join sentences (e.g., "That is the man who lives next door"). Grammar in Use - Pre-intermediate
Describing a background action interrupted by a shorter one (e.g., "I was walking home when it started to rain"). Knowing when to use "I work" (habit) vs
For general truths (e.g., "If you heat ice, it melts "). and that to join sentences (e.g.