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When I'm playing baseball I often forget everything.
Literal
Baseball [obj-を] is-doing-[cond-と] everything [obj-を] forget thing [subj-が] many [explanatory-のです].
The conditional ~と marking habitual co-occurrence ('whenever X, then Y'). 何もかも ('everything, anything and everything') is an emphatic 'everything' form, built on 何 (what) + もかも (every combination). ~ことが多い ('it often happens that...') is a fixed way to express frequency.