The design of mashup tools combines elements from end-user development and software composition in the context of the Web. The challenge for mashup tool designers is to provide end-users with suitable abstractions, programming models and tool support for easily composing mashups out of existing Web services and Web data sources. In this paper we describe the design of a natural mashup composition environment based on the EnglishMash controlled natural language. The environment proactively supports users as they are learning the syntax of the EnglishMash language with features such as auto-completion, immediate feedback, live preview of the mashup execution and component discovery and selection based on natural language descriptions.