Usage of っ inbetween て and た forms?

Question

I have seen quotes in manga and games in which a verb will be presented as (If I remember correctly) "しまっていった" as opposed to "しまっていた".

Why is this? Is this an anomaly? Is one form intentionally used to convey a different meaning? If the latter is the case, what are the differences in meaning?

Answer 1

いた is the past form of いる, and いった is the past form of 行く. They are Japanese subsidiary verbs with completely different functions.

• (-て)いる: progressive form ("is V-ing") or continuation of a resultant state ("has V-ed")
• (-て)いく: physically or psychologically going away from the speaker; gradually over time toward the future; do something and leave

When used as a subsidiary verb, (-て)いく and (-て)いった are usually written in hiragana. See: Difference between -ていく and -てくる

For example:

• 雪は溶けていた。
  The snow has (already) melted.
  _{(Someone only found a wet road and puddles.)}
• 雪は溶けていった。
  The snow (gradually) melted away.
  _{(Someone was observing the melting process.)}