Example:

Verbs also change form depending on who is doing the action or is in the state of being.

Irregular verbs’ roots differ in form when tense or number is changed, and it is always a good idea to check a grammar book for the correct forms of irregular verbs.

What Tense Should I Use to Write My Erotic Story?
As a rule, you want to write in Simple Present and Simple Past Tense using the other tenses sparingly when needed, if at all, and avoiding the Perfect and Perfect Progressive Tenses as they are extremely cumbersome to read. Some writers prefer Past and others prefer Present. Either is fine, but not being consistent with verb tenses will cause problems in your prose. The choice of Past or Present Tense will also affect the feel of your story.

I like Present Tense for short erotica, especially when I am writing in first person, because it feels more immediate to the reader. I am in the minority on this. The events are unfolding as the reader reads them; there is an advantage to this in erotica. You can add tension and create an air of vividness and freshness in a story making it feel more immediate or urgent by writing in Present Tense. But beware: depending entirely on Present Tense instead of a strong story line, tight prose, and actual exciting events to create all of the tension and freshness in your prose will not work.

Present Tense can feel more stylized, because it's uncommon (especially for third-person narratives), but if you're writing in the first person, it can feel quite natural and you can use Simple Past Tense for flashbacks. Simple Present Tense can be difficult to use effectively. Many writers find that they slip in and out of the Past Tense when they are writing in Present. (Good editing can solve this problem on your rewrite.) Some readers don’t feel comfortable reading fiction written in Present Tense.

Past Tense is used to show events that are past. It is also the most commonly used tense in fiction. (Once upon a time, there was a dyke named Sue who lusted after Mary...) Most authors use Past Tense in their work. Many readers feel more comfortable reading Past Tense because they are more familiar with it. One great benefit of writing in Past Tense, especially in first person, is that the narrator can reflect on events as they are told since they have already happened. Time has passed so a first person narrator has had time to reflect on the events and can share insights about them that might seem unrealistic for a Present Tense first person narrator to do. But, if the narrator is first person and the story is in Past Tense, the reader on some level is aware that the narrator is relaying past events and, thus, is a live in most cases. So Past Tense can on some levels limit dramatic tension.

Ultimately whether you use Past or Present Tense is your choice. The story you are telling may demand Past or Present Tense and it is always in your best interest to listen to the story. Good writing requires that the author keeps the verb tenses consistent—using either Present or Past Tense without randomly switching from one to the other. A few writers are able to write one portion in one tense and another portion in another tense, but it takes a very skilled author to do so effectively and correctly. (Practice this skill by observing the works that successfully pull it off and those that don’t and practicing switching tense when there is a logic reason to do so.) A lack of consistency in verb tense causes readers to stop reading because of confusion; however, there are legitimate points in stories where you will need to switch tenses.

For the most part, you will write either in Past or Present Tense. When logic calls for a switch in tense (for example, you are writing in Present and include a description of the events that have already happened), you want to switch tenses. However, if you switch without a logical reason, it is going to confuse your reader and make it hard for them to keep track of the events. Switching verb tenses upsets the time sequence of narration; therefore, you will want to keep the verbs you use in the same tense.

Since changes in verb tense help readers understand the temporal relationships among various narrated events, but unnecessary or inconsistent shifts in tense can cause confusion; generally, writers maintain one tense for the main discourse and indicate changes in time frame by changing tense relative to that primary tense. Occasionally, for dramatic effect, you may wish to narrate an event in present tense as though it were happening now or providing back story in Past Tense so the reader knows it has already happened.

Correct:  
Yesterday we walked to the beach, but later rode the bus back to the hotel.

Incorrect:  
Yesterday we walk to the beach, but later rode the bus back to the hotel.
Yesterday we walked to the beach, but later ride the bus back to the hotel.

Do shift tense to indicate a change in time frame from one action or state to another.