If the situation being described is an ongoing or current one, the present tense is needed, even in a past-tense context: “Last week she admitted that she is really a brunette” (not “was”).
Pairs of verbs that go together logically have to be kept in the same tense. “Patricia described her trip to China and writes that the Great Wall really impressed her.” Since “described” is in the past tense, and the writing contains her descriptions, “writes” should be “wrote.”
Lots of people get into trouble with sentences that describe a hypothetical situation in the past: “If he would have packed his own suitcase, he would have noticed that the cat was in it.” That first “would have” should be a simple “had”: “If he had packed his own suitcase he would have noticed that the cat was in it.” Also, “The game would have been more fun if we had [not “would have”] won.” This sort of construction consists of two parts: a hypothetical cause in the past and its logical effect. The hypothetical cause needs to be put into the past tense: “had.” Only the effect is made conditional: “would have.” Note that in the second example above the effect is referred to before the cause.
Students summarizing the plot of a play, movie, or novel are often unfamiliar with the tradition of doing so in the present tense: “Hester embroiders an A on her dress.” Think of the events in a piece of fiction as happening whenever you read them—they exist in an eternal present even if they are narrated in the past tense. Even those who are familiar with this pattern get tripped up when they begin to discuss the historical or biographical context of a work, properly using the past tense, and forget to shift back to the present when they return to plot summary. Here’s how it’s done correctly: “Mark Twain’s days on the Mississippi were long past when he wrote Huckleberry Finn; but Huck’s love for life on the river clearly reflects his youthful experience as a steamboat pilot.” The verb “reflects” is in the present tense. Often the author’s activity in writing is also rendered in the present tense as well: “Twain depicts Pap as a disgusting drunk.” What about when you are comparing events that occur at two different times in the same narrative? You still have to stick to the present: “Tom puts Jim through a lot of unnecessary misery before telling him that he is free.” Just remember when you go from English to your history class that you have to shift back to the past tense for narrating historical events: “Napoleon lost the battle of Waterloo.”