Verb tense consistency on the sentence level

• Keep tenses consistent within sentences. 

Do not change tenses when there is no time change for the action.
           INCORRECT: 

             
          CORRECT: 

Since there is no indication that the actions happened apart from one another., there is no reason to shift the tense of the second verb.

Note another example. 

         INCORRECT:

         CORRECT:   

The above sentence means that Mary walks into a room at times.  The action is habitual present.  The second action happens when the first one does.  Therefore, the second verb should be present as well.

Change tense only when there is a need to do so.
 Usually, the timing of actions within a sentence will dictate when the tense must change.

           CORRECT: 

 The first action will take place in the future; therefore, the second one will as well.

              CORRECT:                 The second action took place in the past; the first action occurred before the past action.  Therefore, the first action requires the past perfect tense (had + verb).

Verb tense consistency on the paragraph level

• Generally, establish a primary tense and keep tenses consistent from sentence to sentence. 
• Do not shift tenses between sentences unless there is a time change that must be shown.

          PRESENT TENSE PARAGRAPH               All actions in the above paragraph happen in the present except for the future possibility dependent upon a
     present action taking place: " If a cat sees the bird, the cat will kill it."

     PAST TENSE PARAGRAPH
     

     All of the actions in the above paragraph happen in the past except for the possibility dependent upon
     one action taking place:  "If a cat saw the bird, the cat would kill it."