Exegesis:

hē de Mariam ‘but Mary,’ in contrast with pantes hoi akousantes ‘all who heard’ in v. 18.

panta … ta rēmata tauta ‘all these things’; panta is emphatic, as shown by its position.

rēmata ‘things’ (many translators) or ‘words’ (Dutch versions, An American Translation, Zürcher Bibel); the former interpretation implies a reference to all that Mary had seen, heard and experienced and is, therefore, wider in scope than the latter and as such preferable.

sunēterei ‘(she) treasured up’; as compared with the aorist tense in the preceding verse the imperfect tense here suggests that Mary kept her experiences in mind not for a moment but for a long time.

suntēreō ‘to keep carefully,’ ‘to treasure up in one’s memory.’

sumballousa en tē kardia autēs ‘pondering in her heart,’ scil. all these things; the object is to be taken over from sunetērei ‘she treasured up’; en tē kardia may go with sunetērei, or with sumballousa; because of the word order preferably the latter.

sumballō (also 14.31) transitive ‘to bring together,’ hence ‘to compare,’ ‘to interpret,’ or, in a more weakened sense, ‘to ponder.’

Translation:

Kept all these things, pondering them in her heart. In some receptor languages ‘to keep/to treasure,’ in the sense intended here, requires a reference to the place where the things are kept, whereas ‘to ponder’ can go without; hence such transposition of the locative phrase as ‘placed in her heart and considered’ (Tamil, Kannada), ‘she-put in her breath and … she-thought’ (Tboli, similarly in Tagalog). In other languages both verbs have to be qualified; hence a transposition of the object, e.g. ‘kept and pondered all these things in her heart’ (Bahasa Indonesia RC). — To keep, or, ‘to keep in the heart.’ A connotation of secrecy (which can slip in rather easily because of the esoteric tendencies of many religions) must be avoided. Ponder, or, ‘continually think-about’ (Tboli), ‘turn around in the mind’ (Batak Toba), a reiterative form of ‘think’ (several other Indonesian languages), ‘puzzle forth, puzzle back’ (Sranan Tongo).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.