I would like to know if we can change our destiny; and I know there are two types of destiny: a) That which is fixed and cannot be changed, e.g. death, Rizq (sustenance), etc. and b) That which is suspended by actions, e.g. how to do good/bad deeds. I would like to know if our marriage to our partner is decided under type (a)?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Whatever a person does is predestined before he was even born whether it is marriage or something else. However, as you mentioned in the question, the Divine Decree is of two types: Mubram and Mu'allaq. Mubram refers to the inevitable and unchangeable fate, while Mu'allaq refers to that which is in the records of the Angels.

Mu’allaq is the one that is meant in the verse (which means): {For every term is a decree. Allaah eliminates what He wills or confirms, and with Him is the Mother of the Book} [Quran 13:38-39]. Mu’allaq is also the type meant in the narration of the Prophet who said: “Nothing prevents fate except supplication, and nothing prolongs life except good deeds and acts of obedience.”

Therefore, it should be known that marrying a specific person is a Mubram [inevitable] Decree which does not change. So, if Allaah Has written in the Preserved Tablet that a specific man will marry a certain woman, then this is a Mubram Decree and it will inevitably happen.  However, this does not contradict the fact that this decree is conditioned on the occurrence of a reason which is the acceptance of the husband, because Allaah predestines a matter and its reason.

Allaah Knows best.