In short

Pointing words say which one. dieser/diese/dieses (this) decline exactly like the definite article (dieser Mann, diese Frau, dieses Kind, diesen Mann, diesem Bus). The article der/die/das can also stand alone as a stressed demonstrative meaning “that one / him / her / that”: Der gefällt mir! Its forms match the article except dative plural denen and genitive dessen/deren.

The rule

Demonstratives point at something: this book versus that book, or simply this one.

• dieser, diese, dieses = this / these. It is the everyday “pointing” word and takes the same endings as the definite article (a so-called der-word). Whatever ending der/die/das/den/dem has, put it on the stem dies-: dieser Mann, diese Frau, dieses Kind, acc. diesen Mann, dat. diesem Bus.
• jener, jene, jenes = that (over there). It declines like dieser, but in modern German it sounds formal or literary. In normal speech, “that one” is der/die/das da or just dieser.
• The article as a stressed pronoun. The definite article der/die/das can stand on its own, stressed, replacing a noun: Der gefällt mir! (That one I like!), Den kenne ich nicht (I don’t know him / that one), Die da bitte (That one there, please), Das weiß ich nicht (That I don’t know). This is very common in speech. Its forms are the same as the article except: dative plural denen (not den) and the genitive forms dessen / deren.
• dessen / deren before a noun. The genitive forms also serve as possessives in place of sein/ihr: dessen for a masculine or neuter owner, deren for a feminine or plural one. They never take an ending of their own, so an adjective after them declines strongly (dessen neuer Chef). They point back at the noun just named: Paul besucht seinen Bruder und dessen Freundin (the brother’s girlfriend, not Paul’s).
• Stressed vs. unstressed der. The same word der is unstressed as an article (der Mann = the man) but stressed and accented as a demonstrative standing alone (Der ist nett = That guy is nice). Stress and position (usually first in the sentence) tell them apart.
• A few more pointing words: solch- = such (solche Bücher, so ein Tag); derselbe/dieselbe/dasselbe = the same; derjenige/diejenige/dasjenige = the one (who), usually before a relative clause.

Forms & patterns

dieser (this) - declines like the definite article. The same table works for jener and solcher:

der/die/das as a stand-alone demonstrative pronoun (stressed). Differences from the plain article are highlighted:

So only the dative plural (denen) and the genitive (dessen/deren) look different from the article der/die/das/den/dem - everything else is identical.

derselbe (the same) and derjenige (the one who): the first part declines like the article, the second part like a weak adjective (always -e or -en). Written as one word:

Examples

Common mistakes

• diese Mann -> dieser Mann - masculine nominative ends in -er, just like der.
• Ich sehe diese Mann -> Ich sehe diesen Mann - accusative masculine takes -en (like den).
• Mit diese Bus -> Mit diesem Bus - dative masc./neuter takes -em (like dem).
• Ich helfe die da -> Ich helfe denen da - dative plural of the stand-alone demonstrative is denen, not die/den.
• Overusing jener for everyday “that” -> use der/die/das da or dieser - jener sounds stiff and old-fashioned in speech.

Tips

• dieser = a der-word. Copy the article’s ending onto dies-: know der/die/das - den - dem, and you instantly know dieser/diese/dieses - diesen - diesem. The same trick works for jeder, welcher, solcher.
• The stand-alone der/die/das is everyday spoken German for “this one / that one / him / her.” It usually sits first and stressed: Die kenne ich! Don’t mistake it for the unstressed article.
• Memorise the only two surprises: dative plural denen and genitive dessen/deren - everything else matches the article.
• derselbe and derjenige are written as one word, and only the front half carries the case ending: denselben, demselben, derselben.