Praise be to Allah.

Firstly: 

Not paying at all constitutes kufr (disbelief) that puts one beyond the pale of Islam, according to the correct scholarly opinion. For more information, please see the answer to question no. 5208. 

With regard to the person who prays sometimes and does not pray at other times some of the scholars are of the view that he is also a kaafir (disbeliever), which is the view narrated from a number of the Sahaabah, and is also the view cited in fatwas of the Standing Committee for Issuing Fatwas which was headed by Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him). 

Other scholars are of the view that he does not become a kaafir unless he does not pray at all, such as Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) and Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him). For more information, please see the answers to questions no. 52923 and 83165. 

Secondly: 

The scholars differed concerning the one who deliberately does not pray (such as one who is too lazy to pray, and the like): does he have to make up the prayers, as is required of one who sleeps and misses a prayer or one who forgets, and is it in fact more appropriate that the one who does not pray without any excuse should be instructed to make up the prayer, rather than the one who do have an excuse, as is the view of the majority of scholars, and the four madhhabs and others are agreed on this point? 

Or is it the case that he does not have to do that, and if he does make them up it will not benefit him, either because not praying constitutes kufr and the prayer of the disbeliever does not benefit him so long as he remains a disbeliever, and he is not instructed to make up what he missed during the time when he was a disbeliever and apostate; or because every act of worship is connected to a particular time, and if a person does not do it at the proper time with no legitimate excuse, it will not be accepted from him, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever does an action that is not part of this matter of ours will have it rejected” (narrated by Muslim, 1718)? For more information, please see the answers to questions no. 105849 and 197247 

The ruling on shortening the prayer when not travelling is no different from the ruling on not praying at all. If a person offers a prayer but omits one rak‘ah or prostration, or he omits one of its pillars or essential parts deliberately, his prayer is invalidated and he is like one who did not pray it at all. He is closer to toying with the rituals, and such a person is in grave danger, if Allah does not bestow His mercy upon him and bless him by enabling him to repent sincerely. 

It was narrated that Ibn ‘Abbaas said: Allah enjoined the prayer on the lips of your Prophet (blessings and peace of Allah be upon him); four rak‘ahs when not travelling, two rak‘ahs when travelling, and one rak‘ah at times of fear. Narrated by Muslim (687). 

Ibn Hazm (may Allah have mercy on him) said: The number of rak‘ahs does not change (when travelling) except in the case of Zuhr, ‘Asr and ‘Isha’: they are four rak‘ahs when not travelling, for both healthy and sick individuals, two rak‘ahs when travelling and one rak‘ah at times of fear. There is scholarly consensus on all of that, except with regard to whether these prayers may be offered with one rak‘ah at times of fear – there is a difference of opinion concerning that.

End quote from al-Muhalla (3/185) 

Fourthly:

 it is not permissible to put two prayers together without an excuse; if a person puts prayers together without any excuse or legitimate justification for doing so, then he is sinning, because he is going against the shar‘i texts that regulate the prayer, such as the verse in which Allah, may He be exalted, says (interpretation of the meaning): “Verily, the prayer is enjoined on the believers at fixed hours” [an-Nisa’ 4:103]. 

Another such text is the hadith in which the Prophet (blessings and peace of Allah be upon him) said: “Jibreel (peace be upon him) led me in prayer at the Ka‘bah twice. He led me in praying Zuhr when the sun had passed the meridian and the shadow was the length of a sandal-strap; then he led me in praying ‘Asr when the (length of a thing’s) shadow was equal to its height; then he led me in praying Maghrib at the time when the fasting person breaks his fast; then he led me in praying ‘Isha’ at the time when the red afterglow had disappeared; then he led me in praying Fajr at the time when food and drink become prohibited to the one who is fasting. Then the next day he led me in praying Zuhr when the (length of a thing’s) shadow was equal to its height; then he led me in praying ‘Asr when the (length of a thing’s) shadow was equal to twice its height; then he led me in praying Maghrib at the time when the fasting person breaks his fast; then he led me in praying ‘Isha’ when one third of the night had passed; then he led me in praying Fajr when it had grown light. Then he turned to me and said: O Muhammad, these are the times at which the Prophets before you prayed, the time is between each of these two times (for each prayer).”

Narrated by Abu Dawood (393) and at-Tirmidhi (149). Al-Albaani said: Its isnaad is hasan saheeh, in Saheeh Abi Dawood – al-Umm, no. 417 

Ibn Qudaamah (may Allah have mercy on him) said: The Muslims are unanimously agreed that the five daily prayers are to be offered at certain times which are well-known and strictly defined, and several saheeh hadiths have been narrated concerning that.

End quote from al-Mughni (1/224) 

Once this is established, it is not permissible to put two prayers together unless there is a reason to doing so, such as travel, rain, or sickness. If there is no reason for putting prayers together, then you should abide by the original timings for the prayers