Romans 6:8

ει COND δε CONJ απεθανομεν V-2AAI-1P συν PREP χριστω N-DSM
But if we died with Christ
…………..πιστευομεν V-PAI-1P οτι CONJ και CONJ συζησομεν V-FAI-1P
…………..αυτω P-DSM
…………..we believe that we will also live with him

Paul here reiterates what he has already stated in v.5 as well as to draw out the significance of that connection which he follows up in v.9-10. And we see similar usage of tenses - aorist for died and future for live, pointing to the promise of the eschatological resurrection of our lives in Christ.

συν χριστω
This phrase only occurs here in Romans, though the formula is virtually present in certain occurrences of συν-compounds (v.4, 6, 8b and 8:17) . The formula seems to have originated with Paul (Grundmann) and is used most often with reference to fellowship with Christ in the eschatological glory, but it is also used with past tenses with reference to baptism and to that which baptism is the witness. (Cranfield p.312)

πιστευομεν οτι
I am not too sure about the semantics of language then, but the phrase I believe seem to markdown whatever statement that follows. Cranfield says that it is inserted as an emphasis to indicate the personal and inward commitment of the members of the Church to its truth.

και συζησομεν αυτω
Cranfield's take on this is that it refers to the present life, from the whole structure of the argument here and especially from the content of v.11, which it is closely related to. The Christian's present life is to be a life with Christ in the power of his resurrection, a foretaste of the life with Christ in glory, an eschatological fulfilment of the life already begun. (Cranfield p.312-3)

Maeghan