sehen
to see
To perceive with the eyes; to use the ability to see.
Ohne meine Brille kann ich fast nichts sehen.
Without my glasses, I can hardly see anything.

to see
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to see
To perceive with the eyes; to use the ability to see.
Ohne meine Brille kann ich fast nichts sehen.
Without my glasses, I can hardly see anything.

'Sehen' (to see) is a strong verb and changes its vowel in the 2nd and 3rd person singular. 'e' becomes 'ie'. So it is 'du siehst' (you see) and 'er/sie/es sieht' (he/she/it sees).
Siehst du den Vogel dort auf dem Baum?
Do you see the bird there in the tree?
When you observe another person doing an action, you can use 'sehen' (to see) with a second verb in the infinitive without 'zu'. The second verb then comes at the end of the sentence.
Ich sehe die Kinder im Garten spielen.
I see the children playing in the garden.
In conversations, 'sehen' (to see) is often used metaphorically in the sense of 'verstehen' (to understand) or 'erkennen' (to recognize/realize). It is a common response to signal agreement or understanding.
Ah, jetzt sehe ich das Problem!
Ah, now I see the problem!
In the meaning of 'sich etwas anschauen' (to watch something), 'sehen' (to see/watch) is often combined with media or events. Typical phrases are 'einen Film sehen' (to watch a film), 'ein Theaterstück sehen' (to see a play), or 'ein Fußballspiel sehen' (to watch a soccer match).
Wir haben gestern Abend eine interessante Dokumentation gesehen.
We watched an interesting documentary yesterday evening.
When 'sehen' (to see) is used reflexively with 'sich', it means 'sich treffen' (to meet) or 'sich begegnen' (to encounter each other). It is often used for appointments or chance meetings.
Wann sehen wir uns wieder?
When will we see each other again?
To express a personal assessment or perspective, the fixed structure 'etwas als etwas (Akkusativ) sehen' (to see something as something) is used.
Er sieht diese Aufgabe als eine persönliche Herausforderung.
He sees this task as a personal challenge.
In the meaning of 'kontrollieren' (to check), 'sehen' (to see/check) is always used with the preposition 'nach' and the dative. It expresses a quick check.
Ich gehe mal kurz nach den Kindern sehen.
I'm going to quickly check on the children.
The expression 'kann sich sehen lassen' (is impressive / can be shown off) is a fixed unit. It is used to express that a result or achievement is very good and impressive.
Dein neues Auto kann sich wirklich sehen lassen!
Your new car is really impressive!
to meet / to hit
'sich sehen' and 'sich treffen' can both mean that people come together. 'sich treffen' more strongly emphasizes the specific meeting, often with a place, time, or arrangement. 'sich sehen' often sounds a bit more general and personal: for example, you say that you have not seen someone for a long time, even if what you actually mean is that you have not met up. The words are easy to confuse because in everyday language people often say 'Wir sehen uns' (we'll see each other) when they mean a meeting. If you want to emphasize the arrangement or appointment, 'sich treffen' is more precise.

Wir haben uns seit dem Sommer nicht mehr gesehen.
We haven't seen each other since the summer.
Am Wochenende treffe ich mich mit meiner Kollegin zum Kaffee.
At the weekend, I'm meeting my colleague for coffee.