1) …you start dropping the possessive “s,” preferring “the car of Mary” to “Mary’s car.”
2) …you find yourself using substantive adjectives way too much for English to bear.
3) …the lower-case “p” becomes an “r” sound at first glance. On a similar note, an initial “r” receives a rough breathing at the front: “I’m hunting h’rabbits.”
4) …you’re writing something and a) you neglect to dot the “i,” b) replace the “p” with a pi, and c) write v’s instead of n’s.
5) …while conversing in Spanish, you realize that you just talked about “hoi gatoi,” that is, more than one cat.
6) …participles and infinitives take on a life of their own, in ways that they were never meant to in English.
7) …you have to repress an urge to point out the Greek origins of peoples’ names: “Did you know that ‘Alexander’ means ‘defender of men’?”
8) …it just seems natural to start a sentence with “but,” “and,” or “therefore.”
9) …you (SN) recognize that even English nouns (PN) have cases (PA) and sometimes try to parse them (PA) with little or no real success (SD).
10) … “eggs” become “engs” and “angels” become “aggels.”
I still write my lowercase d's as deltas, and there was a period where I kept misreading billboards because of #3 and #4.