Rashi on the Former Prophets

How we should send it back. In what manner should we send it away, so that He will not be angry, and so that His wrath will be removed from us.

A guilt-offering. As an indication that you confess that you have betrayed Him.
Then you will be healed, and you will realize. That He did this, for why wouldn't His hand turn away from you then?

Philistine rulers. They were five, as it is stated, "The Azites, the Ashdodites, the Ashkelonites, the Gittites, and the Ekronites."

Nursing cows. Nursing.
Which have not borne a yoke. All this is for the test. Since these cows are not capable of pulling [a load], and furthermore, they will yearn for their young. If the Ark will have the power to enable them to pull it by themselves, we shall know that He brought this upon us.

In the box. Escrin in O. F.

Confined in the house. 'כָּלוּ' is an expression of 'כֶּלֶא' [imprisonment].

Set on a straight course. This word is a grammatical hermaphrodite. This teaches us that even the young recited a song, "Sing aloud, sing aloud, O Ark of acacia wood! Exalt yourself…" as stated in Maseches Avodah Zorah. According to its simple meaning, 'וַיִשַּׁרְנָה' is an expression meaning 'straight,' i.e. they followed a straight path.
Lowing as they walked. This is an expression of the cry [גְּעִיָה] of cattle.
Beis Shemesh. Yisroel was there.

And they rejoiced to see. They were going to see how it came alone, and out of their joy, they behaved with levity, for they did not gaze at it with awe and respect.

Open village. A village without a wall.
The large rock. This is the large rock previously mentioned, for the 'ל' comes in place of the 'נ' [in the word 'אָבֵל'], similar to 'נִשְׁכָּה' instead of 'לִשְׁכָּה' [chamber], in the book of Nechemyah. And because they experienced mourning there, they called it 'אָבֵל'.

Seventy men, fifty thousand men. Targum Yonoson renders, 'And He killed of the elders of the people seventy men, and of the community fifty thousand men.' Our Rabbis, however said, 'Seventy men, each of whom was equal to fifty thousand, or fifty thousand men each of whom was equal to the seventy of Sanhedrin.'