The imperfective aspect is used in a variety of circumstances where it is felt to imply some specific aspect of on-going activity. While one can easily become overly clever in creating ever longer lists of aspectual connotations, there are some generally recognized connotations that have been identified and given specific names.
The Habitual Imperfect: The Imperfective Aspect, eg. "he was ranting", can in the proper circumstances be felt to imply "he habitually ranted" or "he used to rant"
The Continuous Imperfect: The Imperfective Aspect, e.g., "he was ranting", can also be felt to imply that an action kept on going-on; thus, "he kept on ranting." This implication is made explicit in English by repetition: "he was ranting and ranting".
The Conative Imperfect: The Imperfective Aspect, e.g., "he is running", can be felt to imply "he was trying to run." We still find this "conative imperfective aspect" in English, often in the jargon of sports: "There's the pitch. Conner is stealing second. The throw. They got him!" Here, the statement that "Conner is stealing second" clearly means "Conner is trying to steal second" or "Conner is in the process of stealing second"; it is a conative present (imperfective) in English.
The Incohative Imperfect: The Imperfective Aspect can also be felt to imply that someone is trying to do something. One might interpret the example above ("Conner is stealing second") as meaning "Conner is trying to steal second." This overlap is good to keep in mind, since these designations are really only interpretations in context of what may be felt to be implied by the use of a marked form.