1. Mutation of the first letter. The séimhiu is used to lenite (soften) the first letter of a word when it is mutated. For example, the word "bosca" (box) becomes "an bhosca" (the box) when the article "an" is added.
2. Eclipsis of the first letter. The séimhiu is also used to eclipse (harden) the first letter of a word when it is eclipsed. For example, the word "fear" (man) becomes "an fhear" (the man) when the article "an" is added.
3. Indicating a palatalized consonant. The séimhiu can be used to indicate that a consonant is palatalized. Palatalization is a sound change that occurs when the tongue is raised towards the roof of the mouth. For example, the word "sí" (yes) is pronounced with a palatalized "s".
4. Separating two vowels. The séimhiu can also be used to separate two vowels that would otherwise be pronounced as a single vowel. For example, the word "aoi" (oats) is pronounced with a séimhiu between the "a" and the "o".
These are just some of the most common reasons why a word in Irish might take a séimhiu. There are other more complex reasons why a séimhiu can be used, but these are the most important ones to know.