Rashi on Tanakh1 Samuel 27

Now I might perish one day. Perhaps he will ambush me and suddenly attack me.¹Dovid did not trust Shaul and was convinced that he was laying a trap for him by asking him to remain.—Metzudas Dovid

Why should your servant remain… And be a burden to you.²Probably, Dovid did not want to live with idolaters and therefore asked in a diplomatic fashion, e.g., ‘Why should your servant be a burden to you,’ to settle in a separate city.

Days and four months. The minimum [number] of days is two. It is impossible to explain this word, 'יָמִים' as a year³As we find יָמִים to mean ‘year’ in Bereishis 41:1. because Dovid did not go to Achish until Shmuel had died, and Shaul only reigned after Shmuel's death⁴Therefore יָמִים is to be taken literally, i.e., days. four months, as we learned in Seder Olam. Also [this can be deduced from the following:] the number of Eili's years [forty years which he judged Yisroel],⁵Above 4:18. and the twenty years that the Ark was in Kiryas Ye'arim⁶Above 7:2. total sixty years. Subtract from that Shmuel's fifty-two years, plus one year from his conception, with these four months, plus the seven years which Dovid reigned in Chevron⁷II Shmuel 2:5. before he brought the Ark from Kiryas Ye'arim. This [calculation] discloses that Shaul reigned after Shmuel's death but four months.

Dovid…would go up. This is the present tense, i.e., he was always accustomed doing this.

In the direction of Shur. From the entrance of Shur.

Where did you raid. The 'ל' is interchangeable with a 'נ' [so the word אֶל=אָן which means 'where'], as in "he prepared a large chamber [נִשְׁכָּה],"⁸Nechemyoh 13:7. in the Book of Ezra, meaning the equivalent of לִשְׁכָּה [=chamber].