Krishna releases the sons of Kubera

This story can be found on pg 73 of Swami Tapasyananda's translation of the Śrimad Bhāgavatam, 1981. ed.

From this point on, we introduce the dual form. Unlike most languages that we are used to which have a singular and a plural, Sanskrit has a dual, which refers to 'two'. It is called द्विवचन. It is less common than the other forms, but naturally we have to learn it. This story may seem a bit difficult as compared to what has gone before, but by the time we get through it, all will be clear. 

A few things to note:

1. The ending of the dual noun

2. The form of the verb which agrees with the dual form

3. That all adjectives etc (anything other than an indeclinable) will agree with the noun.  

You are used to the sentence: 

कुबेरस्य पुत्रः गर्वितः अस्ति (Kuber's son is arrogant/conceited)

The dual form is:

कुबेरस्य पुत्रौ गर्वितौ स्तः

पुत्रौ= two sons

गर्वितौ=the adjective agreeing with पुत्रौ. Just like गर्वितः agrees with पुत्रः

स्तः = is the verb 'to be' root अस्, which you are used to seeing as 'asti' (is)

पुत्रौ नारदेन शप्तौ भवतः

The (two) sons are (literally - become) cursed by Narada. 

 नारदेन =by Narada

शप्तौ agrees with पुत्रौ, just like गर्वितौ in the first sentence. शप्त* is past participle from the root शप् , like मृत*  (dead) which becomes मृतः or मृता, depending on the gender of the noun it is describing. You met this in the story of Putanaa. Here too there is agreement - with putrau

भवतः = verb from the root भू, agreeing with the 3rd person, masculine, dual form. So far you have met bhavati, which agrees with Krishna, or putra etc.

पुत्रौ शापेन वृक्षौ प्रविशतः

The (two) sons, by the curse, enter (two) trees

शापेन= by the curse (instrumental case, 3V)

वृक्षौ = two trees, just like putrau - two sons

प्रविशतः = verb to enter (pra+viś). Notice the same ending as स्तः and भवतः

एकदा कृष्णेन नवनीतम् गृहीतम् अस्ति

Once, butter was taken/grabbed, by Krishna 

एकदा means ek baar

Navaneetam is a neuter noun, and griheetam is a past participle agreeing with navaneetam

यशोदा कुपिता भवति

Yashoda becomes angry

सा कृष्णम् उलुखे बध्नाति

She ties Krishna to a mortar

उलुखे is the locative (7V) of ulukhalam (neuter noun). The locative is used to show where Krishna is tied, whereas in Hindi or English we would say he is tied 'to'

बध्नाति is the verb form from the root 'badh' meaning to bind/tie, agreeing with Yashoda

सः वृक्षयोः  मध्ये गछति 

He goes between the two trees

वृखयोः = the dual, locative form. This is completely new, so please pay extra attention to this form.

मध्ये - is the locative 7V of madhyam, which we are familiar with from Hindi

उलुखेन वृक्षौ छिन्नौ भवतः

By the mortar, the two trees are cut (probably translates better as felled in this context), but chinnau is from the root chid, to cut.

वृक्षौ पततः

The two trees fall. 

Note the ending of the verb - patataH, which is the same as bhavataH, staH and pravizataH. (z is श by the Harvard Kyoto transliteration scheme. As I have requested you, we will use either IAST or HK, depending on what keyboard facilities are available. You can find the scheme here: http://www.sanskrit-lexicon.uni-koeln.de/monier/help.html

वृक्षयोः अन्तः पुरुषौ स्तः

Inside the two trees, are two men

वृक्षयोः locative dual 7V

अन्तः inside (indeclinable, so the ending doesn't change with the dual form - remember avavya - the words that don't change their ending) 

पुरुषौ two men

स्तःare - verb agreeing with puruSau

पुरुषौ कुबेरस्य पुत्रौ स्तः

The two men are Kubera's sons

कृष्णेन पुत्रौ मुक्तौ भवतः  

By Krishna, the two sons become free.