English grammar rules. The usage of 'For and 'Since' in English. The use of FOR: We use For when we measure the duration – when we say how long something lasts. For + a period of time To measure a period of time up to the present, we use the present perfect tense and not the present tense. I have known her for a long time. (Correct) I know her for a long time. (Incorrect) And now let's look into the use of Since: The use of SINCE Since gives the starting point of actions, events or states. It refers to when things began. Since + a point in time (in the past), until now. I've been waiting since 7 o'clock. I have known him since January. Enjoy your learning with Italki!