An additional point is that when you have verses with two adjectives and nouns in them the adjectives regularly come in the first half of the verse and their respective nouns in the second half. So
siluestrem tenui musam meditaris auena
is typical. The adjectives set it up, the nouns resolve it. The case endings sort out which adjective goes with which noun. The poets take full advantage of the fact that Latin is an inflected language!
formonsam resonare doces Amaryllida siluas is not actually indirect statement, but the problem is the same: you have two accusatives and you have to decide which one is the object of the infinitive. Here it’s the woods that do the resounding, the echoing, and Amaryllis that is resounded, sounded over and over again. The image is of a love-lorn rustic swain repeatedly calling the name of his beloved. That’s how he “teaches” the woods to echo back his cry.
Note that here again you have the adjective at the beginning, with its noun coming in the second half, and the verb (or in this case verbs) intervening. There’s no room in the line for an adjective for silvas too!
This is pastoral (as silvestris musa signals), a genre that Vergil takes over from Theocritus. Not only the genre, characters too: one of Theocritus’ “idylls” begins:
Κωμάσδω ποτὶ τὰν Ἀμαρυλλίδα, ταὶ δέ μοι αἶγες
βόσκονται κατ’ ὄρος, καὶ ὁ Τίτυρος αὐτὰς ἐλαύνει.
The serenade starts Ὦ χαρίεσσ’ Ἀμαρυλλί,
and Vergil’s line invites us to imagine Tityrus likewise calling “O formons’ Amarylli,” only Vergil puts it at a remove. The form Amaryllida enhances the Greek feel.
The 3rd-foot caesura articulates the verse into two unequal halves (so to speak!). Usually it comes after the first syllable of the 3rd foot, as in
siluestrem tenui | musam meditaris auena
but sometimes a half-beat later, as in
formonsam resonare | doces Amaryllida siluas
Caesura at the end of the 3rd foot is avoided. That would split the verse exactly in half, and would result in both halves taking off from the “longum” of the foot, an undesirable effect.
I’d encourage you to train yourself to read the verses word by word in the order in which the words come (as you should also be doing with Cicero, and with anything else you read), and also to read them metrically without mechanically scanning them first (though you may well need to do that initially). Concentrate on getting to the caesura, and until you get more practiced, bash the metrical beat, 6 per line. (For the latter part work back towards the middle from the end if you have to, the closing —⏑⏑—— .) Then once you’ve got the rhythm firmly fixed in your head you can start giving the words their natural accents, and the dynamic relationship between the word accents and the meter will begin to emerge. It’s one of the many glories of reading Vergil.