De Morgan's law is an equivalence and is usually used as an inference rule, i.e., a way of converting one formula into another. Before you can apply De Morgan's law or any other inference rule, you need to represent the given English statements as logical formulas. For this you first need to determine your alphabet: predicate symbols, constants and possible functional symbols. This in turn depends on the structure of the English statements: what properties and objects they talk about. For example, you should decide whether you need a unary property (predicate) "_ is long", a binary property "_ lasts _ minutes" or a function "length(_)" so that you can express the fact that some film is too long or not long enough. There are also other variants to formalize the concept of movie length.