Languages can change over time. The form of English that you speak is likely to be slightly different to the form of English that your parents — or grandparents — speak. For example, you may pronounce some words differently from them or there might be some words that you've heard of that they haven't (and vice-versa).
Many people underestimate the extent to which languages can change. To give you an idea, here is the Lord's Prayer as it might have been written in English over a six hundred year period:
|Old English (~1000 AD)||Middle English (~1350 AD)||Early Modern English (~1611 AD)|
One way that you can make your conlang more interesting is to give it a history, just like natural languages. There are many things that you can do to add history — or the appearance of history — to a conlang.
Language families and proto-languages
As languages change, the people who speak them sometimes split up into groups and travel apart from each other. Since there is usually very little contact between these groups, the languages that these two groups speak often change in different ways. When these languages change so much that the two groups can no longer understand one another we say that the one language has split into two separate languages and that these two languages are part of the same language family.
Natural languages almost never arise spontaneously from nothing. Usually a natural language is a changed form of some other language and is part of a family. For example, French, Portuguese, and Romanian all come from Latin. English, Swedish, and German all come from a language that we call Proto-Germanic.
When you're reading about the history of languages, you might often come across the term proto-language. A proto-language is just a language that has had new languages come out of it; a parent language, if you like.
There's nothing intrinsically special about proto-languages compared to non-proto-languages. The speakers of Latin and Proto-Germanic didn't know at the time that the languages they were speaking would become proto-languages. They're not neccessarily simpler or more complex than other languages and they don't have any special grammatical features. They were just in the right place at the right time to have children.
Adding history to a conlang
The obvious way to make a conlang with history is to invent a proto-conlang and evolve that proto-language in the same way that a natural language might evolve. When you do that you end up with a language that looks like it has a history because it actually does have a history.
But what if you already have a mostly complete conlang and you want to give history to it? This is much harder because you will have to work backwards, slowly uncoving the proto-language. You'll almost certainly need to revise a whole bunch of stuff as you discover more about your conlang's proto-language.
If that sounds like too much work then there are a few things you can do to give a language the appearence of a history without having to invent a whole new conlang first.
Before we can do anything, we need to know how natural languages can change over time. Let's talk about sound changes.