schließen
to close
To shut an opening such as a door, window, or book so that passage is no longer possible.
Könntest du bitte das Fenster schließen? Es zieht.
Could you please close the window? There's a draft.

to close
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to close
To shut an opening such as a door, window, or book so that passage is no longer possible.
Könntest du bitte das Fenster schließen? Es zieht.
Could you please close the window? There's a draft.

The past participle 'geschlossen' (closed) is often used like an adjective to describe that something is shut. It describes the result of the action 'schließen' (to close).
Alle Fenster sind geschlossen.
All windows are closed.
'Eine Tür schließen' (to close a door) is an everyday, reversible action. 'Ein Geschäft schließen' (to close a business), on the other hand, often means the permanent end of operations. The object of the sentence (door vs. business) reveals the meaning.
Bitte schließ die Tür, es ist kalt.
Please close the door; it's cold.
In the meaning of 'beenden' (to end), 'schließen' (to close) is used mainly for formal occasions such as a meeting, debate, or event.
Der Redner schloss seinen Vortrag mit einem Zitat.
The speaker closed his talk with a quote.
When 'schließen' means 'folgern' (to conclude), it is always used with the preposition 'aus' + dative. The structure is: Aus [information] schließt man [conclusion].
Aus seiner Reaktion schließe ich, dass er überrascht war.
From his reaction, I conclude that he was surprised.
In the meaning of 'eine Vereinbarung treffen' (to make an agreement), 'schließen' is used in fixed combinations. Typical ones are: 'einen Vertrag schließen' (to conclude a contract), 'einen Kompromiss schließen' (to reach a compromise), 'Frieden schließen' (to make peace), or 'eine Ehe schließen' (to enter into a marriage).
Die beiden Parteien haben einen Kompromiss geschlossen.
The two parties reached a compromise.
The reflexive form 'sich schließen' (to close by itself) describes a process that happens without direct human action. It is often used for automatic or natural processes.
Die Blüte schließt sich in der Nacht.
The flower closes at night.
The expression 'jemanden in die Arme schließen' means 'to embrace someone'. The person being embraced is in the accusative.
Nach der langen Reise schloss die Mutter ihr Kind fest in die Arme.
After the long journey, the mother held her child tightly in her arms.
to think
'denken' and 'schließen' can both refer to a conclusion. With 'sich etwas denken', you arrive at an assumption or explanation in your mind, often in a more personal and less formal way. 'aus etwas schließen' means that you logically derive a result from information or signs. The words can be confused because both show that you come to a result. 'schließen' is more precise when there is a reason or proof as the starting point. 'denken' remains more general and subjective.

Aus der kurzen Antwort schließt er, dass sie keine Zeit hat.
From the short reply, he concludes that she has no time.
Daniel sieht die leeren Teller und denkt sich, dass das Essen gut war.
Daniel sees the empty plates and figures that the food was good.