We'll start our exploration of grammar by examining the ways that words can be altered and rearranged to create new meanings. For now, try not to focus on the actual meaning that's being conveyed, but rather on the way that that meaning is constructed.
The morphological ways of marking a word are often referred to under the collective term affixes. There are lots of different types of affixes and we'll look at some of the most common ones here.
Prefixes & Suffixes
Prefixes and suffixes are small word fragments that are added to the beginning or end of a word in order to change its meaning. Prefixes are added to the beginning of a word and suffixes are added to the end.
Prefixes and suffixes are a very important way for a language to change the meanings of words. Almost every language has them to some degree. In English, among other things, we use suffixes to show plurals on most nouns and the past tense on most verbs;
dog with the plural suffix -s becomes dogs walk with the past tense suffix -ed becomes walked
An example of an English prefix would be the negative prefix un-;
happy becomes unhappy
It's also possible to put an affix inside a word; these kind of affixes are called infixes and they're less common than either prefixes or suffixes. For example, the language Tagalog sometimes forms the active voice by infixing the affix -um-:
bili "to exchange something for money" becomes bumili "to buy"
Another example of an infix occurs in the conlang Quenya, where the past tense forms of some verbs are formed by infixing -n- (and also suffixing -e);
tec- "to write" becomes tence "wrote"
Apophony (sometimes referred to as simulfixes) is when you change the sounds that make up a word. The best way to describe apophony is to show you an example. In English we sometimes change the vowels in a noun to show that it's plural; <foot> becomes <feet>, <goose> becomes <geese>, and <mouse> becomes <mice>.
What's happening here is that the vowels are all moving towards the front of the mouth. The change that occurs is always systematic, for example, all the vowels in a word moving to the front (as we see above). If this reminds you of sandhi, it should. Apophony is just a sandhi rule that happens to be triggered by some process happening in the grammar, whereas with sandhi the trigger is purely phonological.
When apophony occurs on vowels it is often called ablaut while on consonants it's mutation.
An example of consonant apophony, or mutation, occurs in the Welsh language. In Welsh, nouns that start with consonants will sometimes change that consonant to a different one when they are preceded by certain grammatical words. Different changes happen with different words and it isn't just grammatical words that can trigger the change. For example, the Welsh word for "stone" is <carreg>, but "the stone" is <y garreg>, "my stone" is <fy ngharreg> and "her stone" is <ei charreg>.
Reduplication is when a part of a word is repeated in order to change its meaning. The affix-y name for this is duplifix. The repeated part of the word can go anywhere around (or in) the original word.
The Indonesian language uses this, often to pluralize things. For example, "sayur" (vegetable) will become "sayur-mayur" (vegetables), or "sayur-sayuran" (vegetables, but can also translate as artificial vegetables).
In English reduplication isn't used very much at all, but one use for it is as an intensifier, to try to make the meaning of a word stronger; "super" is reduplicated to "super-duper". It's also sometimes used in baby-talk.
Function words are basically the syntactic versions of prefixes and suffixes. The difference from them is that function words are whole words that, instead of being fixed to another word, are simply juxtaposed next to it. Function words in English include everything from the articles (a, the), to the prepositions (in, for, over), to the final element in phrasal verbs (to put off, to eat out).
Because they are a syntactic phenomenon, function words don't just act on single words; they can act on whole phrases. For example, the article "the" in the phrase "the house" only affects the one word "house", but the preposition "in" in the phrase "in the house where I grew up" affects the whole phrase "the house where I grew up".
A clitic is sort of like a cross between an affix and a function word; it's physically attached to a word and affects it phonologically like an affix, but grammatically it acts at the level of phrases like a function word.
An example of a clitic in English is the possessive 's (as in the boy's bike). So why is this a clitic and not a suffix? Here's why... suppose that you wanted to express the concept that "the castle" belongs to "the king". You'd say "the king's castle". Now suppose that you wanted to say that "the castle" belongs to both "the king" and "the queen". If the possessive 's was really a suffix then you'd have to say "the king's and queen's castle" (two s'), but you don't, you say "the king and queen's castle" (one 's). This is because the possessive s is a clitic and is capable of governing the whole phrase "the king and queen" rather than just "the king" and "the queen" separately.