http://en.wikipedia.org/wiki/Present_tense

The present tense is the tense (form of a verb) that is often used to express: Action at the present time, a state of being, a habitual action, an occurrence in the near future or an action that occurred in the past and continues up to the present.

 

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/pres.htm

The simple present tense is one of the most common tenses in English. This page explains the rules for forming the tense with regular verbs. There are only two basic forms for the simple present tense; one ends with -s and the other doesn't.

 

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/presnq.htm

Negatives and Questions in the Simple Present Tense. In the simple present tense, negative and question forms are made using the auxiliary verb "do". This page explains the rules. Negatives in the simple present are formed by adding don't or doesn't before the simple form of the verb.

 

http://www.englishpage.com/verbpage/simplepresent.html

The Simple Present express the idea that an action is repeated or usual. The action can be a habit, a hobby, a daily event, a scheduled event or something that often happens. It can also be something a person often forgets or usually does not do. The Simple Present can also indicate the speaker believes that a fact was true before, is true now, and will be true in the future. It is not important if the speaker is correct about the fact. It is also used to make generalizations about people or things.

 

http://www.englishclub.com/grammar/verb-tenses_present.htm

The simple present tense is subject + auxiliary verb + main verb. We use the simple present tense when: the action is general, the action happens all the time, or habitually, in the past, present and future, the action is not only happening now or the statement is always true. There are three important exceptions: 1. For positive sentences, we do not normally use the auxiliary. 2. For the 3rd person singular (he, she, it), we add s to the main verb or es to the auxiliary. 3. For the verb to be, we do not use an auxiliary, even for questions and negatives

 

 

http://en.wikipedia.org/wiki/Past_tense

The past tense is a verb tense expressing action, activity, state or being in the past. In English, there are two distinct types of past tense: 1. Present perfect (see perfect tense)  and 2. Preterite (or simple past). Each of these may also be found in the progressive (continuous) aspect. Simple past is formed for regular verbs by adding –ed to the root of a word. Example: He walked to the store. A negation is produced by adding did not and the verb in its infinitive form. Example: He did not walk to the store. Question sentences are started with did as in Did he walk to the store?

 

 

http://elblogdelingles.blogspot.com/2006/02/lesson-7-past-tense-pasado.html

In English speaking in past we used two types of verbs: The regular verb: They are the verbs finished in “ed”, for example: I worked [aiguorkt]. I worked. I studied [aisstadid]. I studied. Los verbos irregulares: Son los verbos que aparecen recogidos en una lista de tres columnas y cuya pronunciación hay que aprenderse de memoria. Structure: Subject + verb in “Past Tense” + predicate.

 

http://www.englishpage.com/verbpage/simplepast.html

Use the Simple Past to express the idea that an action started and finished at a specific time in the past. Sometimes the speaker may not actually mention the specific time, but they do have one specific time in mind. When you are using a verb tense with only one part such as Simple Past (visited), adverbs usually come before the verb (often visited). Please remember this is different from verbs with more than one part such as Present Continuous.

 

http://www.idiomasgratis.com/ig_view_resource.php?html_code=ig_basico_c2_a4_p1.html

Present tense: This it is a verbal time that is used to express actions that take place in the present and with a determined frequency, that is to say, every day, each morning, etc. Past tense: It is a verbal time that is used to express actions that took place in the past, without it excessively concerns the moment of the past at which they took place.

 

http://esl.about.com/library/beginner/blwas.htm

Page that reflects diverse exercises to understand the past tense in its negative forms, interrogatives etc.