Q.Why are ethers relatively inert compounds?

With no acidic O-H and its lone pairs sterically hindered, an ether offers little for ordinary reagents to grab onto, except under forcing conditions like hot concentrated HI.

Unlike alcohols, ethers have no O–H bond, so they have no acidic hydrogen and cannot be deprotonated or undergo typical alcohol-type reactions (esterification, oxidation to carbonyl, etc.). The C–O bonds themselves are reasonably strong, and the ether oxygen's two lone pairs, though basic in principle, are sterically shielded by the flanking alkyl/aryl groups, making the oxygen a weak nucleophile/base in practice. As a result, ethers are stable towards dilute acids, bases, most oxidizing agents and reducing agents, and mild reagents in general — reacting only under forcing conditions, suc …

Ethers have two lone pairs on oxygen and act as weak Lewis/Bronsted bases. With strong mineral acids (e.g. cold conc. $HCl$, $H_2SO_4$, $HI$), the ether oxygen accepts a proton ($H^+$) to form a protonated, positively-charged oxonium salt, e.g. $R-O^+(H)-R'\ X^-$.

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Anisole undergoes electrophilic nitration mainly at the ortho and para positions because the methoxy group is an o,p-directing activator.

Anisole ($C_6H_5-OCH_3$) has a methoxy group attached to the benzene ring. Even though oxygen is electronegative, the lone pair on the oxygen atom is conjugated (delocalised) into the aromatic ring by resonance, which increases electron density strongly at the ortho and para positions of the ring. This makes anisole highly reactive towards electrophilic aromatic substitution and directs the incoming electrophile predominantly to the ortho and para positions.

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