In the first case, there is kind of ellision of the first part of the sentence, in order to emphasize what is said. The standard sentence would be, more or less: "(El hecho) es que eso es lo que hago": The fact is that THAT is what I do". "Es que" is used in colloquial language: (another examples: "Es que no me hace caso": "But, you see?, he doesn't pay attention to me!"; "¿Es que no me quieres?: "SO, is it true you don't love me?" In the second sentence, SI is used, in this case, to emphasize what is said, and not to initiate a conditional clause. Read the definition of the RAE (http://www.rae.es/RAE): SI: ... 5. conj. Úsase a principio de frase, da énfasis o energía a las expresiones de duda o aseveración. ... ¡SI dije que esto no podía parar en bien! On the other hand, there is an inversion of the logical order of the sentence, to emphasize the number (quince) and the verb. The process would be like that, more or less: Standard sentence: Tengo quince. 1st transformation (emphatical): Los que tengo son quince (more or less litterally: the number of them I have is fifteen) "I DO HAVE fifteen" 2nd transformation (emphatical): Quince son los que tengo."I have FIFTEEN (no more, no less)" 3rd transformation (emphatical): ¡SI quince son los que tengo!. "It's true! Just FIFTEEN!" 4th (adding a contrasting conjunction): Pero si quince... "BUT, believe me!, I got exactly fifteen!" Hope it helped you.:)