1. PostgreSQL 16 Documentation, SQL Commands, INSERT. The official documentation specifies the syntax as INSERT INTO tablename [ ( columnname [, ...] ) ] VALUES ( { expression | DEFAULT } [, ...] ). This confirms that INTO and VALUES are required keywords.
(Source: PostgreSQL Official Documentation, Chapter: "SQL Commands", Section: "INSERT")
2. Ramakrishnan, R., & Gehrke, J. (2003). Database Management Systems (3rd ed.). McGraw-Hill. In Chapter 5, "SQL: Queries, Constraints, Triggers," the basic form for insertion is defined as: INSERT INTO tablename (columnlist) VALUES (valuelist). This is a standard academic textbook for database courses.
(Source: Page 157, Section 5.2.4, "INSERT Statement")
3. Stanford University, CS 145 Introduction to Databases, Course Notes on SQL. The notes on Data Manipulation Language (DML) present the syntax for insertion as INSERT INTO R(A1, ..., An) VALUES (v1, ..., vn);, reinforcing the structure used in the correct answer.
(Source: Stanford InfoLab, SQL DML Notes)