as; in the role of; from the viewpoint of
Marks a role, capacity, or standpoint. Follows a noun or noun phrase.
彼は医者として有名だ。
He is famous as a doctor.
私としては、その計画に賛成です。
As for me, I agree with the plan.
apart from ...; leaving aside ...
Used to change the topic or set something aside, often followed by a new subject. Common in patterns like ～は別として.
冗談は別として、本題に入りましょう。
Joking aside, let's get to the main topic.
even; not even
Used before a negative form to emphasize that something is completely absent. Often follows a counter or a minimal unit like 一人.
一人として来なかった。
Not even one person came.
thinking that ...; trying to ...
Follows a volitional form (～ようとして) to express an attempt or intention. Related to とする.
See also: とする
彼は早く起きようとして、目覚ましをかけた。
Trying to wake up early, he set an alarm.
たる is a stiff, literary form usually fixed in phrases like 教師たる者, while として is the everyday way to express the same idea of role or capacity, as in 教師として.
として means 'as' or 'in the capacity of' and marks a stance without the concessive nuance, while としても adds も to mean 'also as' or 'even assuming that.'
として marks a role or capacity (e.g. 'as a teacher'), while にとって marks the standpoint from which something is judged (e.g. 'for a teacher, this is hard').
として means 'in the capacity of' with no negative judgment, while だてらに always implies the action is inappropriate or unexpected for that role.
Derived from the particle と (quoting/resultative) combined with the conjunctive form して of the verb する (to do). The exact historical development is straightforward.