Subjects and verbs must match in sentences in any language. The conjugation of English verbs is far more complex than in some other languages; romance languages, for example, have set conjugation patterns and few irregular verbs. English, by contrast, has many irregular verbs whose conjugations in past, present and future tenses do not follow a set pattern. A writer or speaker delivering a piece in English must correctly conjugate verbs to match their subjects in sentences. The phrase "I do not know" is correctly conjugated whereas "I does not know" is incorrect; the correct conjugation of the first-person in the present tense is "do" rather than "does." The gender of the person is irrelevant in English verb conjugation.
Sometimes colloquial speech alters the formal rules in a language. In English, one of the more common colloquialisms ignores the rules of adverbs by replacing them with adjectives. While adjectives modify nouns, adverbs modify verbs and often end with "-ly." For example, you will often hear people say in colloquial speech, "I want this bad." The correct construction is, "I want this badly." "Bad" is an adjective while "badly" is an adverb.
Parallel structure is one of the most important elements of English grammatical construction. Complex sentences contain more than one clause; in such sentences, the structure of all clauses must match. For example: "This includes matching subjects and verbs and adjectives and nouns, as well as correctly using adverbs. It also includes understanding parallel structure and avoiding common mistakes such as the misuse of the imperfect subjunctive clause." These sentences employ correct parallel structure. "Matching" and "using" are both gerunds, and "understanding" and "avoiding" are both verbal nouns.
The imperfect subjunctive is another grammatical construction often altered in colloquial speech. The most butchered construction in colloquial English involves the confusion of the verb "to be" in the imperfect subjunctive. Whereas English speakers often say, "If I was..." the correct imperfect subjunctive is, in fact, "If I were..." Another common mistake heard in spoken English is the confusion of the words "have" and "of." In this case, speakers confuse correct phrases such as "should have" or "could have" for incorrect phrases such as "should of" or "could of."