Learn French: S'il n'y avait pas de gravité, tout flotterait. - If there were no gravity, everything would float.

In French second conditional sentences, the si (if) clause always takes the imperfect tense (l'imparfait). Just like in English you say 'If there were' and not 'If there would be', French uses avait. The conditional tense (flotterait) is strictly saved for the result clause.

How does il y avait relate to il y a?

Il y avait is simply the imperfect tense of il y a (there is / there are). On its own, it means 'there was' or 'there were'. When attached to si in this hypothetical sentence, it translates to 'if there were'.

Why do we say pas de gravité instead of pas de la gravité?

In French, when a sentence is negated with ne ... pas, indefinite and partitive articles (like du or de la) change to de. In a positive sentence, you would say il y a de la gravité (there is gravity). Because it is negative, de la becomes just de.

Why is it written as S'il instead of Si il?

The word si drops its 'i' and combines with the next word only when followed by il (he/it) or ils (they). For any other vowel, even other pronouns like elle or on, the 'i' remains intact (for example, si elle).