The language corner: Was he ‘by your side’ or was he ‘by your side’?

Let’s start by writing the prepositional locution: it is not «a lado (a lado de)» but «al lado (al lado de)», since the reference is «the side». This expression indicates that the person or thing mentioned is contiguous, close or in an immediate place. As can be seen, the preposition does not go alone, but rather has the article, but making a contraction (a + el = al).

Let’s continue with the other point of the query. Possessive adjectives (mi, mio, tu, tuyo, su, suyo, etc.) can modify a noun, but not an adverb. With this foundation it is normal to say, for example, “Carlos was by your side”, since “side” is a noun. But it is not correct to say that “Carlos was behind you”, since “behind” is an adverb.

In it Spanish language style book it is indicated that «the combinations of the type of behind you, in front of me, above usetc., have not yet been integrated into the general learned language, so it is preferable to avoid them and use instead the variants with the preposition of: behind her, in front of me, above usetc.”.

In this same work it is pointed out that «in the cases in which the combination with prefixed possessives is possible, such as my Your his, etc., postponed possessives are admitted». Thus, it is appropriate to say by your side, by your side, by your side; by your mistake, by your mistake, by your mistake; against you, against you, against you, etc.

When in doubt with this type of expression, the grammatical category of the head word must be verified: if this is a noun, the sentence will be very well constructed; but if the core word is an adverb, the sentence will be incorrect. (See the examples in the second paragraph.) (F)

SOURCES:

Dictionary of the Spanish language, Department of Spanish up to date (electronic versions) and Spanish language style book (2018), from the Royal Spanish Academy.