bekunden
to declare
To clearly and often officially express a feeling, intention, or opinion.
Mit diesem Schreiben möchte ich mein Interesse an der Stelle bekunden.
With this letter, I would like to declare my interest in the position.

to declare / testify
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to declare
To clearly and often officially express a feeling, intention, or opinion.
Mit diesem Schreiben möchte ich mein Interesse an der Stelle bekunden.
With this letter, I would like to declare my interest in the position.

'Bekunden' (to declare / testify) is an inseparable verb because it has the prefix 'be-'. The prefix therefore remains firmly attached to the verb in all conjugated forms and sentence types.
Er bekundet sein tiefes Mitgefühl.
He expresses his deep sympathy.
This verb belongs to formal language and is mainly used in official, written, or legal contexts. In normal everyday conversation, it sounds very stiff.
Der Minister bekundete den Willen zur Zusammenarbeit.
The minister declared his willingness to cooperate.
'Bekunden' (to declare / express) is often combined with abstract nouns that express an attitude or feeling. Typical phrases are 'sein Interesse bekunden' (to express one's interest), 'seine Absicht bekunden' (to declare one's intention), 'sein Beileid bekunden' (to express one's condolences), or 'seine Solidarität bekunden' (to express one's solidarity).
Nach dem Unglück bekundeten viele Menschen ihre Anteilnahme.
After the accident, many people expressed their sympathy.
The meaning 'aussagen' (to testify) is clear when the context involves a court, an authority, or legal proceedings. Signal words include 'vor Gericht' (in court), 'als Zeuge' (as a witness), or 'gegenüber der Polizei' (to the police).
Als Zeuge bekundete er, was er in der Tatnacht beobachtet hatte.
As a witness, he testified about what he had observed on the night of the crime.
to explain / declare
'erklären' and 'bekunden' can both mean that someone says something clearly. 'erklären' is very common and fits when you make something understandable or officially communicate something. 'bekunden' is used more when someone clearly shows or officially expresses an opinion, a feeling, or an intention. It sounds more formal than 'erklären'. In a legal context, 'bekunden' can also mean to testify as a witness. The words can be confused because both occur in official statements. In everyday language, 'erklären' is usually more natural; 'bekunden' fits better with official or solemn statements.

Der Verein bekundete seine Unterstützung für das neue Projekt.
The association declared its support for the new project.
Kannst du mir bitte erklären, wie dieses Formular funktioniert?
Can you please explain to me how this form works?