How do i know whether it is a 'Present Continuous' or 'Present Participle' For example 1.He is reading a book. 2.When I came, she had left.
Participles are forms of verbs. In English there are two, the so-called Present participle, which always ends in -ing; and the Past participle, which has various forms: most often -ed, but also -en, -t, and sometimes no ending. They have various uses, but can often be used in an adjectival sense, either on their own (eg "the coming year") introducing a clause (eg "A building infested with rats"
Present continuous and Present (or past) Perfect are constructions which use these participles following a verb: most often be or have. These are one of the ways in which the participles are used.
So, if a present participle follows a form of the verb be, it is probably a continuous tense. (There are occasional exceptions. For example He is demanding. is probably an adjectival use: if it were a continuous tense, you would expect an object, eg He is demanding to be heard.)
If a past participle follows a form of the verb have, then it is always (I think) a perfect tense. (eg She has gone away.)
Also note that a past participle can follow a form of be, and is usually passive in meaning: Their victory will be celebrated.