If you have been using another editor such as Eclipse or MavensMate for development in Salesforce, you may want to consider a look at VS Code and CLI. Highlights of the new features include auto-complete, syntax highlighting and a "Replay Debugger". Salesforce is actively developing this functionality and now you can connect it to regular sandboxes, even if you aren't ready to totally jump into DX and scratch orgs. Here's what you need to get started:
Start by downloading VS Code:
Next step is to install the Salesforce CLI:
Add some powerful capabilities with these VS Code Extensions:
Now connect to your existing Sandbox.