English adjectives for being sure or certain about something are often interchangeable, but German expressions like bestimmt, gewiss, and sicher are more specific to the particular context in which they are used. Let's take a look at all three.

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Bestimmt can mean either "definitely"

Du machst Filme und die Leute sagen „Das wird bestimmt so“,

You make films and the people say, "It will definitely be like this",

Caption 34, Berlinale - Schauspieler Jürgen Vogel

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or in the nominalized noun something specific or in particular:

Wenn du mal was Bestimmtes vorhaben würdest...

If you were up to something specific...

Caption 12, Wahlspots - Szenen einer Ehe

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Gewiss can mean "certain" in the standard sense

Die Unterstützung des Schirmherrn

The support of the patron

ist den Fußballfrauen also gewiss.

for the women's soccer [team] is certain.

Captions 13-14, Frauenfußball-WM - Der Bundespräsident am Ball

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or as a suggestion of vagueness or uncertainty, a "certain something":

Aber ein gewisser Druck bleibt trotzdem.

But a certain [amount of] pressure remains nevertheless.

Caption 23, Cro - mit „Melodie“ an Chartspitze

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Sicher can mean "certain"

Ich bin mir sicher, es wird klappen.

I am sure it will work out.

Caption 49, German Intro - Jenny

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but in other contexts it means "safe":

Es is' 'n sicherer Standplatz.

It is a safe location.

Caption 36, Für Tierfreunde - Falknerei Feldberg

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The words bestimmt, gewiss, and sicherlich can be used interchangeably when meaning "of course" or "certainly": Sicherlich / Bestimmt / Gewiss werden wir die Deutschprüfung bestehen. (We will certainly pass the German test.)

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Further Learning
Search for some uses of bestimmt, gewiss, and sicher (sicherlich too) on Yabla German to learn about the ways some of these expressions are used in context.