Maliheh, the way is not entirely clear here, at least to me.
There is a suggestion that, when describing works and expressions, a MADS format record would be more appropriate than MODS.
That leaves MODS to describe manifestations and items. A MODS record could have a relatedItem element with a title sub-element containing the title of a work or expression, and/or containing a link to, or identifier for, a MADS record describing the work or expression. The current list of allowable relations in relatedItem @type doesn't fit the FRBR relationships, so one would have to leave that attribute out... which leave @displayLabel as a way of specifying that this is a relation of a manifestation, say, to a larger work. (I know, we're not supposed to use displayLabel to specify, but sometimes, you use what you got!).
The alternative would be to enter a titleInfo element with @type="uniform". This would align with practice currently in MARC, but as with MARC uniform titles, the problem is separating out the parts of those complex creatures, the uniform titles (whether AACR2 or RDA) that pertain to "Work" and those that pertain to "Expression". And, as with MARC, this gets you into having "mixed FRBR entity description" in the top-level elements of the MODS record.
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There are similar attributes for four entities ine the first goup in FRBR, for example: title. We have title of work, title of expression, etc.. Also there are some instructions for these attributes in RDA.
How could be identified these entities in MODS?
MA.student in LIS
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Head of cataloging department of the central library and documentation center. University of Tehran