If one Davened Maariv early may he still put on Tefillin?
If one Davened Maariv early he is nevertheless to put on Tefillin after Maariv. [However one is to do so without reciting a blessing. Likewise, one must avoid entering into such a situation, and is to a;lways put on Tefillin before Davening Maariv early.]
 Admur 30/5 as explained in Tehila Ledavid 30/3 in his final conclusion that the second opinion and final ruling of Admur here follows the 2nd opinion in Rama Y.D. 196/1, as explained in Shach 196/4, that the second opinion of Rama [of which the Tehila Ledavid concludes is the 2nd opinion of Admur here] rules one may do a Hefsek even after Maariv.
Other opinions in Admur and other Poskim: Some Poskim rule one may not put on Tefillin after Maariv is Davened, even if it is still day, as this causes a contradiction in the days status. [first opinion in Admur ibid; first opinion in Rama ibid; Michaber 30/5] See Tehila Ledavid ibid for an alternative explanation and understanding of the 2nd opinion of Admur, which concludes that according to Admur one may not put on Tefillin after Maariv
 M”B 30/17; Tehila Ledavid ibid in name of Elya Raba
The reason: To suspect for the opinion that rules one may no longer put on Tefillin.
 As if he Davens Maariv first he enters himself into a dispute.