There were two participles in Old English: The present and past. The present participle was approximately equivalent to the Modern English "-ing" form of a verb (as in "The singing person") and the past participle was the form used an a adjective or in a passive verbal construction to show what had happened to someone, like in Modern English "I was killed" or "The song was sung.
In the case of class 1 and 3 weak verbs, all strong verbs, and most or all irregular verbs, the suffix "-ende" was added to the stem of the verb to form the present participle, for example: "singan" - "singende" (in such strong verbs as had a long diphthong in the stem, and the "n" but not the "a" of the infinitive suffix, only "-nde" was added to the stem, for example "sēon" - "sēonde"). In class 2 weak verbs, the suffix "-ende" was added to the stem plus the "i" (or "iġ") of the infinitive suffix, for example: "macian" - "maciȝende". The present participle could be used as an adjective to show that someone was doing something, or usually did something; and as an adjective, it was declined like other adjectives, for example "se singenda mann" - "the singing man". It was also only rarely used to show a continuous or currently happening action, for example, "Iċ eom singende þone sang" - "I am singing the song". Note that when used like this it was not declined as an adjective.
The past participle was formed using a dental suffix for class 1 and 3 weak verbs ("-ed", "-t", or "-d", depending on the verb), and "-od" for class 2 weak verbs. Strong verbs took the suffix "-en" and the appropriate stem vowel change for their strong verb class, for example "singan" - "sungen". As earlier mentioned, the strong verb could be used as an adjective as in Modern English, to show that something had happened to a person (and when used like that, it was declined), or as part of the passive verb contruction "ƿesan/ƿeorðan (past participle" or the active perfect tense "habban (participle)" (note that for non-transitive verbs, it was sometimes "ƿesan (past participle" instead of "habban (past participle)". When used in the perfect construction, it could either be declined or not. If you wanted to deline it, use appropriate number form of the strong accusative adjectival form of the past participle, for example, you could say both "Iċ hæbbe þone mann ȝeslæġen" (I have the man slain) and "Iċ hæbbe þone mann ȝeslæġenne".
Both the past and present participles could have the adjectival suffix "-līċ" added to them, with the resulting word having a meaning like "(verb)-able", as in "ġesewenlīċ" - "visible".