The null subject parameter explains why some languages (Spanish, Italian, Polish) permit sentences like 'Hablas español' (omitting 'you') while others (English, French) require overt subjects. The parameter governs whether verb morphology is rich enough to identify subjects independently, and interacts with other properties like word order freedom and subject-auxiliary inversion patterns.
From your work on movement and transformations, you know that syntactic structures involve more than the words you hear — positions can be filled by empty elements whose presence is inferred from morphological and syntactic evidence. The null subject parameter is the best-studied example of a syntactic parameter: a binary grammatical setting that generates a cluster of surface differences between languages, all deriving from a single underlying property.
The core claim is that languages divide into two types depending on whether they allow pro ("small pro") — a phonologically empty pronoun that can occupy the subject position. In pro-drop or null subject languages like Spanish, Italian, Japanese, and Turkish, the verb morphology is rich enough to uniquely identify the grammatical person and number of the subject. Spanish *hablas* can only mean "you speak" (second person singular); no other reading is possible. Because the verb form itself carries enough information to recover the subject's reference, an overt subject pronoun (*tú*) is optional. English verbs do not have this property: *speak* is compatible with *I*, *you*, *we*, and *they*, so the subject pronoun cannot be omitted without loss of referential content. English requires an overt subject — even in expletive constructions like *It is raining* and *There is a problem*, where the subject position must be filled by a semantically empty placeholder.
The parameter is interesting not just because it explains the presence or absence of overt subjects, but because it predicts a cluster of correlated properties. Languages that allow null subjects also tend to permit free inversion — placing the subject after the verb (*Habla María* "speaks Maria / Maria speaks" in Spanish) without the subject-auxiliary inversion that English uses for questions. They also show that-trace effects differently: English disallows *"Who do you think that _ left early?"* (where the empty subject trace follows *that*), while Italian and Spanish allow equivalent constructions. These correlations suggest that null subject status is a coherent parameter of Universal Grammar rather than an arbitrary collection of unrelated facts.
The acquisition of the null subject parameter is itself illuminating. Children acquiring Spanish begin producing null subjects earlier than children acquiring English produce overt subjects, suggesting the parameter is set quickly once positive evidence arrives — a single clear instance of a null subject licenses the pro-drop setting. Cross-linguistic typology adds further complexity: Japanese and Chinese allow null subjects but for different structural reasons than Spanish (without the rich agreement morphology), suggesting that "null subject language" may be a surface property that multiple parameter settings can produce through different underlying mechanisms. This is a productive area of ongoing research at the intersection of syntax and typology that you can explore once you understand parameter-setting in acquisition.