Yellow Belt
The present perfect, conditionals, reported speech, and the modals of obligation and advice.
Finished past time vs. unfinished time and present relevance.
For marks a length of time while since marks a starting point.
Completed action vs. action in progress / interruption.
Past states and habits, and what would cannot do.
General truths vs. real future possibilities.
Unreal or hypothetical present and future situations.
Predictions vs. intentions vs. fixed arrangements.
Personal obligation vs. external necessity vs. advice.
Prohibition vs. absence of obligation.
Which verbs are followed by -ing and which by to.
So + adjective/adverb vs. such + (a) noun phrase.
Too means "more than wanted" vs. enough means "sufficient", and the word order changes.
Expressing equality and inequality with as ... as.
Choosing who/which/that and knowing when that can be left out.
Tense backshift and pronoun/time-expression changes.
Negative (few/little) vs. positive (a few/a little) meaning.
Common collocations and fixed expressions decide which verb you use.
The ending changes the meaning — how a person feels vs. what causes the feeling.
Two-item quantifiers and verb agreement.