Today we will look at the form श्वशुर्यम् mAs from कथासरित्सागरः 16.1.60.

ततः श्वशुर्यं शोकार्तं स्नेहात्पार्श्वस्थितं तदा ।
गोपालकं स वत्सेशो बाष्पकण्ठोऽभ्यभाषत ॥ १६-१-६०॥

Translation – Then that king of Vatsa with throat half-choked with tears, addressed his afflicted brother-in-law Gopālaka, who remained at his side out of affection (60).

श्वशुरस्यापत्यम् (पुमान्) = श्वशुर्यः – father-in-law’s son = brother-in-law
In the verses the विवक्षा is द्वितीया-एकवचनम्। Hence the form is श्वशुर्यम्।

(1) श्वशुर ङस् + यत् । By 4-1-137 राजश्वशुराद्यत्‌ – To denote the sense of अपत्यम् (descendent), the तद्धित: affix ‘यत्’ may be applied optionally following a syntactically related पदम् which ends in a sixth case affix and has either ‘राजन्’ or ‘श्वशुर’ as its base.
Note: First the सूत्रम् 4-1-83 प्राग्दीव्यतोऽण् prescribes the default affix ‘अण्’ which is over-ruled by the affix ‘इञ्’ (prescribed by the सूत्रम् 4-1-95 अत इञ्) and finally the affix ‘यत्’ prevails because of the specific mention of ‘श्वशुर’ in 4-1-137.
As per the सूत्रम् 4-1-92 तस्यापत्यम् – Following a syntactically related पदम् in which the सन्धिः operations have been performed and which ends in a sixth case affix the तद्धिता: affixes already prescribed (by the prior rules 4-1-83 प्राग्दीव्यतोऽण् etc), as well as those that are going to be prescribed (by the following rules 4-1-95 अत इञ् etc), may be optionally applied to denote the sense of अपत्यम् (descendant.)

(2) श्वशुर ङस् + य । अनुबन्ध-लोपः by 1-3-3 हलन्त्यम्, 1-3-9 तस्य लोपः।
Note: The letter ‘त्’ in the affix ’यत्’ is for the purpose of indicating the intonation (ref: 6-1-185 तित्स्वरितम्)।

Note: ‘श्वशुर ङस् + य’ gets the designation प्रातिपदिकम्‌ by 1-2-46 कृत्तद्धितसमासाश्च। This allows 2-4-71 सुपो धातुप्रातिपदिकयोः to apply in the next step.

(3) श्वशुर + य । By 2-4-71 सुपो धातुप्रातिपदिकयोः – A सुँप् affix takes a लुक् elision when it is a part of a धातुः or a प्रातिपदिकम्।

Note: The अङ्गम् ‘श्वशुर’ gets the भ-सञ्ज्ञा by 1-4-18 यचि भम् which allows 6-4-148 यस्येति च to apply in the next step.

(4) श्वशुर् + य । By 6-4-148 यस्येति च – When the letter ‘ई’ or a ‘तद्धित’ affix follows, the (ending) इ-वर्ण: (letter ‘इ’ or ‘ई’) or the (ending) अवर्ण: (letter ‘अ’ or ‘आ’) of the अङ्गम् with the भ-सञ्ज्ञा takes elision (लोपः)।

= श्वशुर्य । The प्रातिपदिकम् ‘श्वशुर्य’ declines like राम-शब्दः।

(5) श्वशुर्य + अम् । By 4-1-2 स्वौजसमौट्छष्टाभ्याम्भिस्ङेभ्याम्भ्यस्ङसिभ्याम्भ्यस्ङसोसाम्ङ्योस्सुप्‌।
Note: 1-3-4 न विभक्तौ तुस्माः prevents the ending letter ‘म्’ of the affix ‘अम्’ from getting इत्-सञ्ज्ञा।

(6) श्वशुर्यम् । By 6-1-107 अमि पूर्वः – In place of a preceding अक् letter and the following vowel (अच्) belonging to the affix ‘अम्’ there is a single substitute of that preceding अक् letter.