If you've ever learnt another language such as French or Spanish, the concept of tenses is probably old news to you. However, over and over again I see people getting their tenses mixed up. This is very distracting to the reader, so I'll go over what tenses are and how to use them correctly.
A 'tense' tells the reader when an action takes place. For instance, there is a difference between, I walk, I walked, I am walking, I will walk, I will have walked, etc.. While writing a story, you should always keep a constant tense.
The tenses used in story telling are past and present tense.
Here is an example of writing in past tense:
Sally walked to the store and got three apples, but at the register she noticed that one of the apples had been bruised during shipment.
"I'm going to get a new apple before I pay," Sally said to the cashier.
Notice that the verbs 'walked', 'got', 'noticed' and 'said' are all in past tense. 'had been' is in a tense called 'past perfect', which means that this action happened before another action in the past. In other words, the apple 'had been' bruised BEFORE Sally picked it up.
Also, notice that within the dialogue Sally speaks in present tense (using the future tense). Dialogue is generally in present tense if the character is talking in real time, even if the rest of the story is in past tense.
However, if within the story Sally is talking about something that happened in the past, she will speak in past tense.
Past tense is a commonly used verb tense in storytelling. It was used in Harry Potter, The Lord of the Rings, and Percy Jackson and the Olympians. It simulates the narrator telling a story of something that happened in the past.
Let's look at the same snippet in present tense:
Sally walks to the store and gets three apples, but at the register she notices that one of the apples has been bruised during shipment.
"I'm going to get a new apple before I pay," Sally says to the cashier.
Notice that the verbs previously in past tense are now in present tense. These verbs are, 'walks', 'gets', 'notices', and 'says'. Also, past perfect changed to present perfect so that it is now, 'has been'. As well, keep in mind that the dialogue didn't change, as depending on what they're saying, dialogue can be in any tense.
Present tense isn't used as commonly as past tense, but it is becoming more popular in modern books. The Hunger Games was written in present tense, and it can make your writing seem more in the moment.
The main thing you have to remember when writing is to always keep a constant tense in narration. You can't say 'Sally says' and then 'Sally said' in the same sentence. It just doesn't make sense! So choose to use either past tense or present tense and stick with it. I suggest not writing stories in different tenses at the same time, because I guarantee that you will accidentally write in present tense when it's supposed to be past tense and vice versa.
If you need more help, go to http://www.englishpage.com/verbpage/verbtenseintro.html
P.S. I can't tell you how to conjugate verbs into past and present tense. With English especially, you just have to read and speak a lot so that it ingrains in your brain. Plus, I've only ever studied conjugation of French verbs. If you are not a native English speaker and would like swappers to watch for verb tense mistakes, ask them to give you the correct spelling of the conjugated verb! Or, if you are unsure of how to conjugate a certain verb, go to http://conjugator.reverso.net/conjugation-english.html to get all the conjugations of any verb.