Many languages, in order to distinguish between the past, present and future, have something called "verb tenses". Trouble occurs when someone makes the common amateur writing mistake of unintentionally shifting to a different tense. Most commonly, this involves shifting between past and present tense, and seems to most often come up in describing actions after a piece of dialogue.
'''NOTE:''' On a site such as this one which is largely concerned with the discussion of fiction, it is important to note that '''events which take place in a fictional timeline should properly be phrased exclusively in the present tense, although events that take place in the real world in relation to them may be in the past or future tense as necessary.''' For example, "Yesterday I watched the episode of ''Series/StarTrekTheNextGeneration'' in which Picard is assimilated," or, "Next season on ''Series/GameOfThrones'' we will finally get to watch as Joffrey gets his comeuppance." ''See also:'' Administrivia/HowToWriteAnExample.
For example, when I finish writing it, and press the 'save' button, this sentence will be written on this screen in future tense. Ordinarily, however, as I am now doing, I would write this sentence in present tense. Just as while I had written the previous sentence in present tense, this sentence was written in past tense. Unless there is some special reason - a flashback or someone describing something that had happened, in which case past tense is used; or if someone is describing an expected future event, then future tense should be used - writing is usually done in present tense.
If a less experienced writer inserts a quote in past tense here, like "He trashed the place!", [[SelfDemonstratingArticle they tended to fall]] into the past tense afterward, copying the tense of the quote.
More often, the issue is flipped the other way; the characters are speaking in the present tense while the story is being told in the past tense, and the writer has trouble switching from the immediacy of the dialog to the narration perspective, so you got things like this:
--> He looked up. "The sky is falling!" he says.
The correct tense usage would be:
--> He looked up. "The sky is falling!" he said.
--> He looks up. "The sky is falling!" he says.
People tend to relate experienced events in the past tense (as one would expect), but relate events they are creating in the present tense (as they are creating them). This switch in tenses can sometimes be used to judge the veracity of witness statements. It also explains why authors might drop into the present tense when they are writing a piece of the story that they hadn't planned out to begin with.
Oddly enough, the above description fits Japanese prose writing pretty well; states of being are generally described in the present tense if they are in the "present" of the narrative. (Singular acts, such as dialogue, are generally reported in the past tense.) It also has no "tense agreement" rule for embedded sentences, so that dependent clauses need not take the same tense as that of the overall sentence: the sentence "She thought that he was in the kitchen" would have been more literally rendered as "She thought that he '''is''' in the kitchen". Of course, attempts to use the Japanese language in English-language FanFic usually results in a [[GratuitousJapanese completely different trope]]...
Also note that English is rather strict when it comes to the correct use of tenses; closely related languages like German and Dutch have somewhat more relaxed (some would say 'inconsistent') rules.
In English, as in many Indoeuropean languages, an additional confounding factor is the notion of verbal ''mood'', which unfortunately shares similar patterns of conjugation as verbal ''tense''. Most writing in English is in the ''Declarative'' mood, and follows the normal rules for tense conjugation. However, if a person wishes to convey possibility, desire, or something contrary to fact, they might use the ''Subjunctive'' mood, which if used properly can look like hideously incorrect usage of the normal declarative mood. Constructions in the subjunctive mood sound like "if I ''were''... then I ''would be''", and so forth. The Subjunctive is most common in a First Person Narrative, when the character is reflecting, remembering, or giving an opinion. [[note]]Although, English - luckily enough - is rather loose with the use of the subjunctive (some would say [[RunningGag 'inconsistent']]) when compared to languages such as Spanish. For example, ''if I was going to'' is used as frequently as the grammatically-correct ''if I were going to''.[[/note]]
Confusion can also arise with the ''perfect'' tenses (which are not technically tenses of their own, but an ''aspect''). The past tense "He saw them in the kitchen" and the past-perfect "He had seen them in the kitchen" mean subtly different things. The past perfect serves as a "double past-tense", for talking about things that happened before the events being discussed, which are themselves in the past. For constructions that ought logically to use a triple-past-tense, English grammar shrugs and breaks its own rules: 'She thinks he did it', and 'She thought he had done it' but 'She had thought he had done it'. The narration is in the past; from that point, she 'had thought' something previously; at that previous point, she thought he 'had done it' at some even earlier point. Tenseption.
Another common tense issue is the progressive aspect. "Doing X, he did Y" meant that he did X and Y at the same time, not that he did X ''followed by'' Y. For the latter, you would say "Having done X, he did Y" (if X and Y are related actions), or "He did X, then he did Y" (if they aren't). For example:
--> Grabbing his helmet, he walked to the motorcycle.
...is correct; he grabs the helmet while walking. However,
--> Opening the garage door, he raced into the street.
...is not; these two things don't happen at the same time. The right way to say it would be:
--> He opened the garage door and raced into the street.
This mistake is described in the Website/TurkeyCityLexicon under "Not Simultaneous".
Related to this trope is TimeTravelTenseTrouble, where tense confusion is caused by a conflict in the chronological order of history versus the order in which the character(s) or audience experienced it.