Welcome to the site, Aleksandra. This is the right place for questions! The key to knowing which tense to use this: You need to ask yourself whether the time period is 'closed' or open'. Closed time periods are finished, fixed firmly in the past, so we use the past simple; open time periods are unfinished, which means they refer to time up to (and sometimes including) the present moment, so we use the present perfect. Another thing to remember is that the events and details of finished time periods (using the past simple) cannot change, whereas those of unfinished time periods (using the present perfect) may still change. Now take a look at this, and bearing in mind what I've written above, try to decide WHY each tense is used with each time period, and why some can be used with either: up to now = nearly always pres perf (but could be past perf, for example in a story) in the last century = pres perf, if you mean 1919 - 2019. ( Or past, if you mean e.g. 'during the 20th century). during President Kennedy's lifetime = past for three months = could be either (this is a duration, not a point in time) three months ago = past recently = is often (but not always) pres perf - could be any tense this is the first time = usually pres perf ( as in "This is the first time I've been here") lately = usually pres perf throughout the 17th century = past since the II World War = pres perf today = could be either within the last months (note spelling) = usually pres perf, but past is possible When you've got your head around all that, you can try to think of some examples. Good luck!