* All parts of speech can be modified by other words: For example, even a noun like "cat" can be modified: "fluffy cat", "the cat", "that cat".
* There are some words that are almost always unchangeable: These are often considered to be closed classes of words, meaning they don't easily get new members. Examples include:
* Prepositions: (on, in, at, to, from)
* Conjunctions: (and, but, or, so)
* Determiners: (the, a, an, this, that, some, any)
* Pronouns: (I, you, he, she, it, we, they)
However: Even these words can have some limited modifications:
* Prepositions: "onto, inside, outside, throughout"
* Conjunctions: "although, because, however, therefore"
* Determiners: "all, both, many, few, several"
* Pronouns: "myself, yourself, himself, herself, itself, ourselves, themselves"
The important takeaway: While some words are less likely to be modified than others, it's more accurate to say that there are degrees of modifiability rather than parts of speech that are absolutely unchangeable.