Grammatical tense refers to the different forms of verbs that are used to indicate the time a certain action took place or will take place, in written and spoken language. There are three basic tenses, and these are past, present, and future. These can then be broken down into four different forms: simple, progressive, perfect, and perfect progressive. Sometimes the word "continuous" is used instead of "progressive," but these both refer to the same thing. It is important to understand grammatical tense for writing and speaking, to ensure subject/verb agreement. Though most native speakers of a language will just do this naturally, it can become challenging in certain situations, and those learning a language for the first time will need to study the verb conjugations in the different tenses.
Past, present, and future tenses are fairly self explanatory; past tense refers to something that happened already, present refers to something that is happening right now, and future refers to something that will happen. The simple forms of past, present and future is the most basic. In English, "I am," "I was," and "I will be," represent each grammatical tense of the simple form for the verb "to be." It gets slightly more complicated when considering the progressive or perfect forms of verbs.
The next grammatical tense to consider is the progressive or continuous tense. This describes action that is ongoing. For instance, "He is examining," "He was examining," or "He will be examining," are examples of the progressive form for each tense in English. Keep in mind that different languages often handle these concepts with widely varying structures. The perfect form describes an action that has been completed, and is usually formed by adding "had" or "have" to the past participle of the verb. To use the same example, "He has examined," "He had examined," or "He will have examined" are all examples of the various perfect forms.
The perfect progressive, or perfect continuous, grammatical tense is a combination of the two, and describes an action that is still ongoing, but which will be finished. It combines "had/have been" with the present participle form. So, this would be "He has been examining," "He had been examining," or "He will have been examining." It does come in handy to understand these grammatical tenses and their names in a native language, because it can make learning the verb conjugations and sentence structure in a second language a bit easier.