3. "Jemand" and "Niemand"
The indefinite pronouns "jemand" someone" and "niemand" are only used in singular. They change based on case, but not based on gender. As the subject, they conjugate the verb based on the third person singular.
Jemand kann mich sehen. Someone can see me. (Nominative)
Ich kann jemanden sehen. I can see someone. (Accusative)
Ich bringe niemandem etwas mit. I bring no one something. (Dative)
4. Plural indefinite pronouns
Some indefinite pronouns can be used in the plural as well. They conjugate the verb in the third person plural form. These are:
"alle-" (all/everyone/everything), "einige-" (some), "mehrere-" (several), "manche-" (some) and "viel-" (many)
Some of them can also be used in such a way that they take singular endings, usually when they mean "-thing" (everything, something, etc). When used as "everything/something", it takes neutral endings.
Alle können Deutsch sprechen. Everyone can speak German (nominative, plural)
Ich sehe einige. I see some. (accusative, plural)
Er hat mit allem recht. He is right about (with) everything. (dative, singular)
The indefinite pronoun "jede-" when used in the singular, follows the above declension table. If used in the plural, it changes to "alle".
6. Indefinite articles
Indefinite articles can be used to refer back to something previously mentioned or to indicate something like "one (of those)'. Here we use the indefinite article and its case and gender specific declension table.
Ein Mann sitzt im Cafe. - Ja, da sitzt einer.
A man sits in the cafe. Yes, there is one sitting there.
In this example "einer" refers back to "ein Mann", which is a masculine, singular noun in the nominative.
Autos fliegen nicht. - Doch, da fliegt eins!
Cars don't fly. Sure, there's one flying there!
In this example, "eins" refers to "das Auto", which is a neutral, singular noun in the nominative.