For the first time pupils get acquainted with the unions at elementary school. Later, when they start studying the structure of compound and complex sentences, they will need to learn to distinguish types of the unions and also - the unions from connective words (pronouns and adverbs).
First of all, it is necessary to know that the unions are auxiliary parts of speech. They do not answer any questions, unlike independent parts of speech, and have no independent value (sign, action, a state, etc.). The unions are necessary to connect homogeneous parts or simple sentences in difficult. For example, in the offer "The woods, fields and meadows are covered with a snow cover" the union "and" connects uniform subjects of "field" and "meadow". And here in the offer "The woods, fields, meadows are covered with a snow cover, and the winter comes into the own" the union "and" connects simple sentences as a part of compound. The unions are divided into two main groups: coordinating (and, or, or, and and, but, but, however, etc.) and subordinative (that, that, if, because, etc.). Remember that coordinative conjunctions are necessary also for communication of homogeneous parts at transfer, and in compound sentences to connect two independent simple sentences. And here subordinating conjunctions and connective words help to attach a subordinate clause in complex. Learn to distinguish subordinating conjunctions from connective words (pronouns and adverbs). Connective words perform work of the unions, however they answer any question, have a certain value and carry out some syntactic role, as well as any other independent part of speech (adjective, a verb, a pronoun, etc.). So, in the offer "I knew that today there will be for lunch" a word "that" there is a connective word since it is a subject, answers the question "what?", specifies regarding. And here in the offer "I knew that I will not be in time to the station" the word "that" is a subordinating conjunction. It has no certain value, is not a sentence part but only attaches a subordinate clause (izjyasnitelny) to the main clause. Keep in mind that all coordinative conjunctions belong to three groups: connecting (and, not only - but also and - in value and), dividing (or, or) and adversative (but, but, and and - in value but). Besides, the unions can be simple (consisting of one word) or compound (to include two and more words). For example, in the offer "I could not come to them on a visit as did not calculate the time" the union "as" is subordinative and compound. And in the offer "The winter was cold, and we seldom got out to mountains" the union "and" is coordinating, connecting and simple.