Here’s a notion that pops up in SLA and is supported by my classroom observations: less common forms stand out and so are often picked up in acquisition before more common and regular forms. It has been observed that English learners acquire irregular verbs before regular ed past tense verbs. In my Spanish classes, a lot of my students spoke Spanish at home and I noticed those whose Spanish was marginal produced subjunctive forms routinely rather than the called-for indicative. It also makes testing for benchmarks, etc. misleading b/c the tests no doubt favor regular forms but the student may have picked up irregular forms e.g. salga vs sale. One student nails most of the items b/c he’s picked up the regular e.g. indicative form which is represented most on the test whereas another kid gets lots of points for getting the subjunctive. Delving into the acquisition process requires very subtle research but so much of it seems geared toward finding the best way to get students to score well on grammar tests.