Some of the easiest lessons to learn and teach are plural noun rules. Students learn them fast because there is one general rule that the teacher writes on the board: adding "s" at the end of singular nouns. Except for a few nouns that change their forms in the plural (e.g., "man" to "men") most others take on an "s" with slight modifications in spelling (e.g., strawberry/strawberries, potato/potatoes, wife/wives, etc.).
Teachers should make it clear to learners of English as a Second Language (ESL) that adjectives do not change, whether they are modifying singular or plural nouns, feminine or masculine people, objects, animals, or abstract nouns. You can say "nice picture," "nice dogs," "nice girl," "nice boy" or "nice thought" without changing the form of the adjective. Similarly, the adverbs of manner are invariable. They characterize a verb and they are formed by adding "ly" to the corresponding adjectives (e.g., "nice girl/nicely done"). However, because every rule has at least one exception, students should know that the adverb corresponding to the adjective "good" is "well."
Another easy grammar lesson relates to the present tense of English verbs. Students find it easy because they don't have to remember different verb forms for different subjects. They just need to be aware that subjects that refer to pronouns in the third person singular (he/she/it) take an "s" and all the other subjects use the short infinitive form of the verb ("he/she/it knows," but "I/you/we/they know"). The difficulty, especially for ESL students, is to identify the correct personal pronoun to replace names or pronouns in order to use the correct form of the verb ("Mary and I" vs. "Mary and we").
An easy rule to teach students is that in most sentences the word order follows the pattern subject/verb, followed by other parts of speech. For example, "John reads every day" or "The Empire State Building is in New York City." Except for cases when the speaker wants to emphasize a certain word or part of a sentence (as in, "Little did she know about that accident."), word order remains the same in simple sentences. Similarly, word order in "yes/no" questions follows the structure: auxiliary verb/subject/verb. For example, "Do you know her?" or "May I take your coat?"