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The species which donates lone pair of electrons is called a Lewis base.

The species which accepts lone pair of electrons is called a Lewis acid.

Lewis acid and base react to form an adduct or complex.

${\mathrm{BF}}_3$has vacant p-orbital so that it acts as Lewis acid and accepts electrons from the Lewis base. While oxygen of tetrahydrofuran contains two lone pairs of electrons. It donates that lone pair of electrons and acts as Lewis’s base. So in this reaction tetrahydrofuran and ${\mathrm{BF}}_3$react with each other and resulting in the formation of an acid-base complex or adduct. The reaction is as follow;

The species which donates lone pair of electrons is called a Lewis base.

The species which accepts lone pair of electrons is called a Lewis acid.

Lewis acid and base react to form an adduct or complex.

In this reaction, lone pair of oxygen abstracts a hydrogen atom from HI so that${\mathrm{I}}^{-}$ forms a further attack on the carbocation which results in the formation of the following product.

The species which donates lone pair of electrons is called a Lewis base.

The species which accepts lone pair of electrons is called a Lewis acid.

Lewis acid and base react to form an adduct or complex.

In this reaction trimethylamine act as a Lewis base so that a lone pair of nitrogen abstract acidic H-atom of carboxylic acid forms acid-base adduct as shown in the reaction.