
gewiss
certain
Means sure, unquestionable, or established. It expresses a high degree of certainty.
Nach dieser Leistung ist ihm der Sieg so gut wie gewiss.
After that performance, victory is as good as certain for him.
certain
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certain
Means sure, unquestionable, or established. It expresses a high degree of certainty.
Nach dieser Leistung ist ihm der Sieg so gut wie gewiss.
After that performance, victory is as good as certain for him.

certain
Used before a noun to refer to a person or thing that is not specified in detail or is not meant to be named; a particular one.
Ein gewisser Herr Schmidt hat heute Morgen für Sie angerufen.
A certain Mr. Schmidt called for you this morning.

certainly
Used as an adverb to emphasize a statement; certainly, definitely.
Das wird gewiss eine Herausforderung, aber wir sind gut vorbereitet.
That will certainly be a challenge, but we are well prepared.
'Gewiss' (certain) is closely related to the verb 'wissen' (to know). The basic idea is that something is 'gewusst' (known) and therefore certain. The word family also includes 'die Gewissheit' (certainty) and 'sich vergewissern' (to make sure).
Ich muss mich erst vergewissern, ob die Information stimmt.
I first have to make sure whether the information is correct.
The meaning of 'gewiss' (certain) changes significantly depending on its position in the sentence. If it comes after a verb (predicative), it means 'sicher' (certain/sure). If it comes before a noun (attributive), it means 'ein bestimmter' (a certain/particular).
Der Erfolg ist ihm gewiss.
Success is certain for him.
In the meaning 'sicher' (certain/sure), 'gewiss' is often used with the verb 'sein' (to be). The person for whom something is certain is in the dative.
Mit deiner Hilfe ist uns die Unterstützung gewiss.
With your help, support is certain for us.
When 'gewiss' (certain) comes before a noun, it expresses that a person or thing is not known in detail or is not meant to be named. It is declined like a normal adjective.
Er hat ab einem gewissen Alter graue Haare bekommen.
He started getting gray hair after a certain age.
As an adverb, 'gewiss' (certainly) means something like 'sicherlich' (certainly) or 'bestimmt' (definitely) and strengthens a statement. It often appears at the beginning or in the middle of a sentence.
Das wird gewiss nicht einfach, aber wir schaffen das.
That certainly won't be easy, but we can do it.
sure / safe
Both words express that something is true without a doubt. „Sicher“ is neutral and very common in present-day standard German, while „gewiss“ often sounds more elevated or solemn. Use „sicher“ in everyday statements and „gewiss“ when you deliberately want the statement to sound more formal.

Die Nachricht ist gewiss richtig.
The news is certainly correct.
Die Nachricht ist sicher richtig.
The news is certainly correct.