Concentrate, concentrates, concentrated, concentrating

The Simple Past Tense, also called Simple Past or Preterite is a verb tense that is used to describe completed action happend in the past. It is the most basic form of past tense and it is formed by adding d, ed or ied to the base structure of the verb.

Present Perfect Continuous is a tense in the English language used to emphasize that one action is still continuing in the present moment. The difference between Present Perfect and Present Perfect Continuous is that Present Perfect is emphasizing the action of the sentence while Present Perfect Continuous is emphasizing the duration of that action.

A transitive verb always has a noun that  receives the action of the verb, called the direct object.

An intransitive verb never has a direct or indirect object. Although an intransitive verb may be followed by an adverb or adverbial phrase, there is no object to receive its action

Present Perfect is a tense used to express the activity that started in the past and is still going on in the present tense. In British English this is the most used tense of them all. When you want to tell about your experience up to the present, you use Present Perfect. The prepositions that are often used with this tense are NEVER, EVER, JUST, ALREADY, YET and many others.

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