to register / notice
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to register
To enter yourself or someone else officially in a list or register, for example for a course, an event, or with a public authority.
Um an der Konferenz teilzunehmen, müssen Sie sich online registrieren.
To take part in the conference, you must register online.
Verbs ending in '-ieren' are often borrowed from other languages. They are always weak verbs and form the past participle without the prefix 'ge-'. So it is 'hat registriert' (has registered), not 'hat geregistriert'.
Ich habe mich für den Newsletter registriert.
I registered for the newsletter.
You can recognize the meaning from the context. If it is about signing up, it is often used reflexively ('sich registrieren' (to register oneself)). If it is about perception, a subordinate clause often follows. In a technical context, the subject is usually a device.
Bitte registrieren Sie sich am Empfang.
Please register at reception.
In the meaning 'sich anmelden' (to sign up/register), 'registrieren' is used reflexively with 'sich'. To state the purpose of the registration, German often uses the preposition 'für' followed by the accusative.
Ich möchte mich für den Marathon registrieren.
I would like to register for the marathon.
When 'registrieren' means 'bemerken' (to notice), the object is very often a subordinate clause. This is typically introduced by 'dass' (that) or a wh-question word (how, what, when).
Sie registrierte mit Erstaunen, dass ihr Kollege eine neue Frisur hatte.
She noticed with astonishment that her colleague had a new hairstyle.
to recognize / realize
'erkennen' and 'registrieren' can both mean that you perceive or notice something. 'erkennen' often goes one step further: you identify something or understand what it means. 'registrieren' more often means that you take note of something, sometimes only briefly or without reacting right away. Learners may confuse the words because both can fit with information, changes, or signals. In addition, 'registrieren' has other meanings: you can officially register yourself, or a device can record data. These meanings have nothing to do with 'erkennen' as understanding or identifying.
Er registriert beim Arzt kaum, wie nervös er vor der Untersuchung ist.
At the doctor's office, he barely notices how nervous he is before the examination.
Sie erkennt ihre Nachbarin im Park sofort, obwohl sie eine Mütze trägt.
She recognizes her neighbor in the park immediately, even though she is wearing a hat.