natalis: That is, “dies natalis”; we will find out by line 2 that it is Sulpicia's, not Cerinthus's, and that it is “invisus” not because she is concerned about her age but because of how she is to spend the day.
rure: Ablative. The locative would not fit here because its last syllable is long: rūrī. molesto: modifies the nearer noun, “rure”, not “Cerintho” in the next line![2] Cerintho: This is the first mention of his name in the poems. We do not know who he was. Sulpicia names him twice, here and in 5.1, both times in a rather negative way. Here, she refers to their potential separation; in poem 5, she is accusing him of insensitivity. Sulpicia does not address Cerinthus by name as affectionately as Catullus does Lesbia in 5.1, “Vivamus, mea Lesbia, atque amemus”; on the other hand, she never has occasion to refer to her lover as bitterly as Catullus does in 58.
[4] frigidus: Figurative; the river in question is the Arno, which is not notably cold.
Arretino agro: Ablative either of place where or of separation. Arretium is a town in Tuscany, presumably near Messalla's villa.[5] Messalla: The other key figure in Sulpicia's life; see the introduction.
studiose: Note the quantity of the final e, by which you can determine whether this is an adverb or an adjective in the vocative. mei: Objective genitive with “studiose”.[6] non tempestivae viae: The “viae” are “intempestivae”; in other words, Sulpicia says “non volo iter nunc facere.”. The verse is rather corrupt; the correct text might also be “non tempestivam sic properare viam”.
propinque: Kinsman, someone "near and dear".[8] arbitrio meo: The genitive could also have been used here. The ablative is similar to the idiom “sua sponte”; it may also have a legalistic sound.