Learn French: Tu as besoin de ma calculatrice ? Je te la prête volontiers. - You need my calculator? I willingly lend it to you.

When using double object pronouns, there is a strict order. Pronouns representing the first or second person (me, te, nous, vous) always come before the third person direct objects (le, la, les). So "to you" (te) must go before "it" (la).

How do we know to use la instead of le or lui?

la is a direct object pronoun replacing ma calculatrice. Because calculatrice is feminine, we must use the feminine pronoun la to mean "it". lui would only be used for an indirect object (meaning "to him" or "to her"), but here the indirect object is already te.

Why is there a de after besoin?

In French, the concept of "to need" is expressed with the fixed phrase avoir besoin de, which translates literally to "to have need of". The de is a mandatory part of this expression and links it to the thing you need.

Could I place volontiers before the verb, like the English "I willingly lend"?