What I call "TIMELINE" -guide on how to choose the correct verb tense.

When we write or speak, we use all the verb tenses, not one at a time. However, we tend to follow a coherent TIMELINE, that is, verbs that are at the same level of sentence organization, tend to have the same verb tense, unless there is a marker ( adverb, time complement...)

Issues to take into account:

• When we speak about general topics (which are the same in the past, present and future), we use the simple present.
• When we are talking about the time of speaking or an extended time of speaking (that is, not only now, but these six weeks in opposition to the next or past six weeks), we use the present continuous. Markers: now, right now, at the moment, this term, this year, this week...
• When we are talking about things that are finished in the past, we use the past simple. Also when we are talking about things that happened one after the other, or actions that interrupt another action in progress (this one in the past continuous) Markers: ago, last week, last year, in+year, in a time clause when the main clause is in the past coontinuous...When we are talking about simultaneous actions in the past, we use the past continuous, as well as if we want to describe a scene in a narration. We use the past perfect simple when we want to talk about something that happened before something else in the past. Markers: ever, never, already, just, still, yet, for and since, also with the first time... When we are talking about routines or habits in the past, we use the structure used to, we can also use the modal would if we talk about actions and not states. If the action we are talking about started in the past but is somehow related to the present we have to use the present perfect. Markers: already, just, still, yet...if we wan to emphasize the progress, we use the present perfect continuous.
• For the future, we also have several uses which you can see in this post.Markers: tomorrow, next week, next month

In order to practice all this, go to these places:

• Complete revision, tenses chart and exercises at English page
• .Exercise at Dave's Esl Café
• Verb tense quiz 1 and 2 at Efl.net

I think this is enough for the moment, remember, if you have any problems, you can always contact me, write  down your doubts while doing the exercises.