彼は私の肩を軽くたたいた。

he; him; his; boyfriend

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

I; me; private affairs; personal matter; secrecy; selfishness

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

shoulder

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

light (i.e. not heavy); feeling light (i.e. offering little resistance, moving easily); light (i.e. of foot); nimble; agile; non-serious; minor; unimportant; trivial; slight; small; gentle; soft; easy; simple; indiscriminate

to strike; to clap; to knock; to dust; to beat; to play drums; to abuse; to flame (e.g. on the Internet); to insult; to use up money; to call; to invoke (e.g. a function)