Question: Why when you forget to say Birkas haTorah ahava rabba can count as saying Birkas haTorah only if you learn immediately after davening but let say you slept the whole entire day and didn’t say Birkas haTorah but davened mincha/maariv early with a minyan before it was dark, then the Shema counts as learning after Ahavas olam?
A. The Mishna Berura 47:7:13 says the blessing of ‘Ahavas Olam’ takes care of birkas haTorah if you learn immediately after davening without any interruptions. This applies in the morning (for Ashkenazim we say ‘ahava rabba’) or in the evening, for example if someone took a few hour nap in tbe day according to the opinion you have to birkas HaTorah and even those that are lenient it does not hurt to have in mind birkas haTorah when saying ‘ahavas olam’ during maariv.
B. The Mishna Berura (17) says that the reason why you have to learn right after davening is because it’s not apparent you are saying ahava rabba for the mitzva of learning Torah since it’s being said during another mitzvah, of davening. Rav Elyashiv adds that even though Shema can be considered Torah learning but because it’s being said for another mitzvah of krias Shema, which is a mitzva to be recited in the morning and at night once it is dark and the stars are out, and the bracha at the end of ahava rabba does not talk about learning Torah rather the love of Hashem for the Jews then without learning right after davening it’s not apparent it was said for Birkas haTorah.
Answer: The Dirshu (note 22) quotes a Teshuvas Hisorirus that because you recited Shema of maariv too early and it would have to be repeated once it got dark then it is apparent that the paragraph of Ahavas olam was said for Birkas haTorah and the Shema was used for learning.