One of the grammatical tools that Sanskrit makes use of is the समासः - the combination of two or more words to form a single word.
In the case of a tubelight, two words दण्ड and दीप have come together to form दण्डदीपः - 'a light like a stick'.
When creating words to fit modern concepts, grammatical rules for समासः must be adhered to.
In the conjugation of verbs, and in the declension of nouns, pronouns or adjectives, the letter 'स्' is sometimes introduced as in verb-forms like पठसि and वदसि or in noun-forms like मालासु ।
When the 'स्' is preceded by the स्वरs अ or आ - then the 'स्' remains unchanged as in the aforementioned examples पठसि, वदसि and मालासु ।
But if 'स्' is preceded by a स्वर other than अ or आ - that is by इ, ई, उ, ऊ, ऋ, ॠ, ए, ऐ, ओ, or औ - then the स् is converted into a ष् ।
And so we have the सप्तमी विभक्ति बहुवचन forms of तासु, मालासु , तेषु and देवेषु and verb-forms like करोषि and शृणोषि ।
The word अपठत् is a verb form and used in the active voice sentence construction.
The word पठित is an adjective and is used in the passive voice. बालकः पुस्तकम् अपठत् - the boy read a book बालकेन पुस्तकं पठितम् - the book was read by the boy
Yes, you may substitute one for the other if you change the voice of the sentence.
Whenever two words are placed in close proximity, there are chances of a sandhi taking place when alterations take place in the letters to ease pronunciation.
When a word ending with the first letter of a varga (क् , च् , ट्, त् , प्) is followed by a word whose first letter is a स्वर (a vowel) or a मृदु व्यञ्जन (soft consonant), then the letters- क् , च् , ट्, त् , प् are converted into the third letters of their own varga.
So क् , च् , ट्, त् , प् are respectively converted into ग्, ज्, ड्, द् and ब् ।
In case the letters are followed by a स्वर, the letters first get converted and then a further combination takes place when they join with the स्वर
For example- तत् + अस्ति= तदस्ति
Here is an example of - त् followed by a मृदु व्यञ्जन - भगवत् + गीता = भगवद्गीता क् followed by a मृदु व्यञ्जन - वाक् + देवी = वाग्देवी