It is a well-known cliche that you are judged by the way you write and speak. (Cliches, by the way, are true more often than not). Mastering English, proper English, allows a person to communicate more effectively.
It is not always essential to know the structure, terms, and rules that govern grammatically correct communication in English. Most people have some ideas about what is correct simply by listening or reading what is said. A phrase or sentence that sounds awkward is likely not proper English. Nevertheless, knowing and understanding grammar rules can help people avoid common mistakes that could identify them as uneducated or careless. This can make a huge difference in human relationships both on and off the job.
Very common mistakes quickly sort the quick from the not-so-quick. A prime example is the use of "of" in the place of "have" as a helping verb. For example, "I should of seen it coming." Of is a preposition; "have" is the appropriate, and obvious choice in this circumstance.
In English, we speak and write sentences. Each one contains a complete idea. It is broken down into two parts. The first is a subject which is comprised of a person, place or thing which is acting or being. The usually subsequent predicate provides the action or state of being, formed by a verb, and describes the object upon which the action is performed. Of course, it is a bit more complicated than this there are indirect objects, subordinate clauses and more.
The sentence structure is comprised of individual words which are classified into eight parts of speech - each with its own function. These parts include: nouns, verbs, adjectives, adverbs, pronouns, prepositions, conjunctions, and interjections.
For both writing and speaking there are more rules and cases. Some of the articles below will help to address them.
Decades and Centuries explains how to use and punctuate decades and centuries.
Words That Do Not Rhyme
How to Form the Possessive Case
The Wine Dark Sea