The duality (polarity) principle in English Grammar
This principle might be called dualism or dichotomy instead of duality
What I here call the duality or polarity principle implies the existence of a dual or a polar mode. The phenomenon of duality permeates nearly everything in the world. There is life vs. death, female vs. male, regular verbs vs. irregular verbs, stative verbs vs. dynamic verbs, gerund vs. infinitive, simple vs. continuous aspect, analogue vs. digital. You can go on and on drawing examples from all walks of life infinitely. In fact the words <and> and demonstrate this relationship. There is a protagonist and an antagonist. Even God must have an adversary, namely the devil. This duality could be a simplification but crucial for making meaning.

Some of Ferdinand de Sausseure’s (a Swiss philologist 1857-1913) most central ideas are also expressed in pairs of concepts as: diachrony vs. synchrony; langue vs. parole; signifiant vs. singnife´; syntagmatic vs. paradigmatic; even Chomsky generative approach (Competence and Performance) is also derived from this dichotomy. Although this duality is based on polarity, it is in the end a dependency relationship. You can’t know what’s good if you don’t know what’s bad. So “bad” makes “good” possible. IT or computer language (digital language) is also based on two modes (two digits: there is current vs. there is no current). In addition, if I say I have to say born to. The duality system is widely distributed in nature. Furthermore, often the pairs merge or fuse into one and go back to their analogous origin where there are no clear-cut distinctions but a continuous flow.

Since Language is a product of human mind and perception this duality can be used in explaining English grammar particularly the English tenses which is a pitfall for some typical mistakes. Learning or teaching grammatical phenomena should be based on comparison. The past tense is a closed system because the action is complete. The markers like yesterday, last, ago show this clearly. The present or future by contrast is an open system. There are also two aspects. There is a simple and a continuous aspect in all tenses. There are rules / routines / facts (simple) vs. exceptions / current or temporary projects / change (continuous). Past simple can be learned in comparison as follows:

• Past simple vs. past continuous
• Past simple vs. past perfect simple
• Past simple vs. present perfect simple

Verbs undergo the same principle because the verb is the basis for tenses. There are stative verbs (know, understand) vs. dynamic verbs (live, work). Some verbs have both a stative and a dynamic meaning (have). It is stative when it means possession and dynamic when it means experience). Stative implies no change and anything which is dynamic changes. There are regular and irregular verbs (work and go); linking verbs that take adjectives and action verbs that take adverbs or verbs which can be a link or an action depending upon the meaning: He went crazy. He went crazily. She turned quick She turned quickly. There are main verbs and helping verbs; there are transitive and intransitive verbs. There are also two kinds of decisions: decisions which are made on the spot (instant decisions: (will) I am cold I’ll get a pullover) and decisions which are planned: (going to): I am going to see a film this evening)

Infinitive can be contrasted with gerund and present participle. Infinitive looks forward in time i.e. prospective (anticipative) view: (remember (1) to post (2) the letter, whereas gerund looks back in time i.e. retrospective view: I remember (2) posting (1) the letter yesterday. I stopped (2) smoking (1): (stop: give up). I stopped (1) to smoke (2) a cigarette (stop: halt). Thus the difficult job of learning lists of verbs by heart will be a bit easier. The duality or comparison principle can be applied as well to the following areas listed alphabetically:

Adjectives vs. adverbs
Capital letters vs. small letters
Countable vs. uncountable nouns
Definite article vs. indefinite article
Direct speech vs. indirect (reported speech)
Passive vs. active voice
Singular vs. plural
Style (formal vs. informal)
Vowels vs. consonants
Pronunciation vs. Spelling

English (on account of its history) is a language of doubles (Germanic (have) vs. Romance (possess). Most phrasal verbs are Germanic. Words of Germanic origin are short (mono-syllabic) whereas words of Romance origin are usually multi-syllabic: beautiful, comfortable....Words of Germanic origin have a higher frequency than those coming from French. Those of Germanic origin refer to the basic daily life needs: sun, bread, water. All of them are similar to the present day German words whereas words of French origin are cultural. They have to do with education and being well off. The following list shows clearly this duality of the rich and the poor, of farmers and aristocrats, of educated and uneducated: