English traditionally uses very few diacritical marks compared to other European languages.
English employs a huge number of loanwords, however, so to provide enough characters to represent all commonly used English words, you may as well use the entire Latin-1 character set. We would not want, for example, to deprive people of proper orthography for their
Certain romanization systems require still further characters. For example, the capital of North Korea should technically be rendered as P'yŏngyang and the lingua franca of Malawi is technically Chicheŵa.
As you can see, where diacritics remain in use, they are mostly retained from their original languages, and are especially retained where they help avoid confusion with another word. For example, façade is fading in favor of facade, but maté and résumé are still going strong.
Traditionally, the diaresis (¨) is used to indicate that two adjacent vowels should be pronounced separately instead of as a dipthong, as in naïve. This usage has been fading fast, with The New Yorker among the few famously clinging to the old convention.
Otherwise, diacritics are largely found in poetry as scansion markers. Most commonly, the grave accent (`) indicates that a vowel should be pronounced where it is ordinarily suppressed; for example, blessèd and learnèd have two syllables whereas blessed and learned have one, but such examples are only really found in poetry. Similarly, the acute accent (´) indicates which syllable should receive stress, the breve (˘) shortens a syllable, and the macron (¯) lengthens it, but these are almost never encountered elsewhere outside of loanwords.