Under normal circumstances, a Normal Summon will be in face-up attack mode, and a Set will be in face-down defence mode.
Some cards can change this, such as Light of Intervention, which prevents monsters from being set, but allows them to be Normal Summoned in face-up defence mode.
Special summons are different - the default is face-up attack or face-up defence position. Again though sometimes individual card effects can 'special set' the card, or restrict it to a specific mode.
Chat with our AI personalities
Yes. Once per turn, a monster may switch from face-up attack mode to face-up defence mode, or vice-versa, as long as the monster was not summoned that turn, and had not declared an attack. If the monster is face-down, then turning it face up into attack mode is a Flip Summon, which can't be done on the turn the monster was Set from hand.
No, you do not 'tribute' the Synchro Material for a Synchro Summon. They are sent to the graveyard, not tributed. You can tribute an opponents synchro monster for "Assault Mode Activate!" to summon the assault mode monster from your deck.
Defence Mode, or Defence Position, means that the card is placed Horizontally in the monster zone. It could also be Set (face down defence position) or face-up due to being Set and attacked, or switched while face-up from attack to defence.
Nothing, unless the card specifically mentions a penalty for not attacking. If a monster is in defense mode it is not able to declare an attack, and there are no consequences for not attacking unless the monster's text mentions one.
only with a cards effect. during a normal battle, you can't just simply set it.