Is it popular to contract "this is" to "this's"? Or is it better to keep the full form?
- This's where we'll go tomorrow.
- This is where we'll go tomorrow.
When we use ['s] for has or is, this represents a contraction in the pronunciation. Instead of saying has or is as a separate word with its own vowel, we reduce the word to just /s/. This /s/ gets attached to the previous word. The result is that we lose a syllable in the pronunciation:
In the first sentence Ben is represents two syllables. But in the second sentence Ben's represents just one syllable.
However, we cannot always reduce the auxiliaries has or is to just /s/. The reason is that English has a rule that we cannot put a suffix /s/ after another s-like sound. S-like sounds are called sibilants.
The English sibilants are /s, z, ʃ, ʒ/. The first three are the sounds at the ends of the words yes, jazz, fish. The last one, /ʒ/, is the middle consonant in the word vision. It occurs at the ends of some unusual words like rouge, mirage and beige, which we borrowed from French.
The special compound sounds (affricates) /tʃ, dʒ/ also finish as a sibilant sound. These affricates are found at the end of the words watch and judge.
When we have a word like has or is after a sibilant sound, we cannot reduce the word to just /s/. We must keep the vowel in the word:
For this reason you will not see has or is represented as ['S] after sibilant sounds in normal writing in published books or on notices. However, in very informal writing, such as in text messages, people may often misrepresent is or has as ['S] even after sibilant sounds. This is just to save time when writing. It does not represent a contraction in the speech.
This's does not exist (certainly not in British English or any English dialect I am aware of). Always use this is.
This is an exception word as far as I know and I've never heard of this's although I do know it applies to other words in speech such as running is so tiring>running's so tiring and to names in writing and speech such as Ben is here> Ben's here. (Canadian Native English Speaker btw).