Describe your changes in imperative mood, e.g. "make ABC do something" instead of "[This patch] makes ABC do something" or "[I] changed ABC to do something", as if you are giving orders to the codebase to change its behavior.
So you will find a lot of Git commit messages written in this style. If you're working on a team or on open source software, it is helpful if everyone sticks to that style for consistency. Even if you're working on a private project, and you're the only one who will ever see your git history, it's helpful to use the imperative mood because it establishes good habits that will be appreciated when you're working with others.