1. A method comprising: receiving a text item input by a user to an authoring environment presented on a user device, the text item including one or more words forming a beginning portion of a sentence; determining, by operation of a system having one or more computers, a plurality of text strings using text included in search results received from a search engine, each text string including a matching portion and one or more suffixes, wherein the matching portion matches the text item, and wherein each respective suffix is located after the matching portion; ranking, by operation of the system, the one or more suffixes based on a credibility score calculated for each suffix and a frequency score calculated for each suffix, wherein the credibility score of a particular suffix indicates an estimated credibility of a source of the text string including the suffix, the frequency score of a particular suffix indicates an estimated frequency of appearance of the suffix; and providing one or more of the suffixes based on the ranking for display on the user device as one or more corresponding suggestions for completing the sentence begun by the user input to the authoring environment.