The study of Latin, the basis for the so-called Romance Languages ("Romance" meaning "from the Romans") was a great help to me in mastering Spanish and French. English, my native language, has many, many Latin-based words in its vocabulary (for example, "populace," "scrupulous," and others); but English lacks much of the grammar structure that assists in learning Spanish and French.
Take verb conjugations, for example. We conjugate verbs in English, of course, by assigning person, tense, and number to the verb. We say,"I see, you see, I saw, you saw, I have seen," etc.; but none of that is much help in Spanish and French where verbs are inflected (given a different form) by a generally consistent spelling pattern (for regular verbs, of course) to give the verb the proper inflection, depending on the tense, person and number.
In Latin, we learn "amo, amas, amat..." a consistent way to conjugate the verb "amare," meaning "to love." "Amare" is the infinitive form (meaning "to love"), "amo" is first person singular meaning "I love," and so forth through all the conjugations. In Spanish, the same approach applies. The infinitive is "amar." The conjugation is is "amo, amas, ama...," etc. It is similar in French. What we learn about Latin verb conjugation is transferable to Spanish, French, Italian, Roumanian, Portuguese, etc.
Also, in Latin, we learn how to decline nouns. A noun's declension is its form (spelling) according to how it is used in a sentence. For example, the Latin sentence "Video puellam," means "I see the girl." On the other hand "Puella amat puerum," means "The girl loves the boy." In the former example "puellam" is in the objective (object of the sentence) case; in the latter it is in the nominative (subject of the sentence)case. Depending on the noun's position in the sentence it is has a different ending; it is declined.
We don't do much declension in English, except for nominative and objective case pronouns (I to me, he to him, etc.), and we don't encounter it too much in Spanish and French, except, again, for pronouns. Spanish and French do require adjectives that accompany nouns to have matching spelling endings; for example "caballo rapido" (fast horse - masculine singular) or "muchachas bonitas" (pretty girls - feminine plural). However, if you're going to study German or Russian, you will encounter noun declensions galore.
My studies in Latin, with its difficult grammar, verb conjugations, and noun declensions, helped me greatly in mastering the less complicated, but similarly patterned Spanish and French languages. In fact, I would recommend anyone considering the study of linguistics to take at least two semesters of Latin.